Etext of Principles of Physiological Psychology 
by Wilhelm Wundt translated by Edward Bradford Titchener 
1904

Introduction
Part I. The Bodily Substrate of the Mental Life
Chapter 1. The Organic Evolution of Mental Function
Chapter 2. Structural Elements of the Nervous System
Chapter 3. Physiological Mechanics of Nerve Substance
Chapter 4. Morphological Development of the Central Organs
Chapter 5. Course of the Paths of Nervous Conduction
Chapter 6. Physiological Function of the Central Parts

Principles of Physiological Psychology 

INTRODUCTION

1. The Problem of Physiological Psychology

THE title of the present work is in itself a sufficiently clear
indication of the contents. In it, the attempt is made to show the
connexion between two sciences whose subject-matters are closely
interrelated, but which have, for the most part, followed wholly
divergent paths. Physiology and psychology cover, between them,
the field of vital phenomena; they deal with the facts of life at
large, and in particular with the facts of human life. Physiology is
concerned with all those phenomena of life that present them
selves to us in sense perception as bodily processes, and
accordingly form part of that total environment which we name the
external world. Psychology, on the other hand, seeks to give
account of the interconnexion of processes which are evinced by
our own consciousness, or which we infer from such
manifestations of the bodily life in other creatures as indicate the
presence of a consciousness similar to our own.

This division of vital processes into physical and psychical is
useful and even necessary for the solution of scientific problems.
We must, however, remember that the life of an organism is really
one; complex, it is true, but still unitary. We can, therefore, no
more separate the processes of bodily life from conscious
processes than we can mark off an outer experience, mediated by
sense perceptions, and oppose it, as something wholly separate and
apart, to what we call 'inner' experience, the events or our own
consciousness. On the contrary: just as one and the same thing,
e.g., a tree that I perceive before me, falls as external object within
the scope of natural science, and as conscious contents within that
of psychology, so there are many phenomena of the physical life
that are uniformly connected with conscious processes, while these
in turn are always bound up with processes in the living body. It is
a matter of every-day experience that we refer certain bodily
movements directly to volitions, which we can observe as such
only in our consciousness. Conversely, we refer the ideas of
external objects that arise in consciousness either to direct
affection of the organs of sense, or, in the case of memory images,
to physiological excitations within the sensory centres, which we
interpret as after-effects of foregone sense impressions.

[p. 2] It follows, then, that physiology and psychology have many
points of contact. In general there can of course be no doubt that
their problems are distinct. But psychology is called upon to trace
out the relations that obtain between conscious processes and
certain phenomena of the physical life; and physiology, on its side,
cannot afford to neglect the conscious contents in which certain
phenomena of this bodily life manifest them selves to us. Indeed,
as regards physiology, the interdependence of the two sciences is
plainly in evidence. Practically everything that the physiologists
tell us, by way of fact or of hypothesis, concerning the processes in
the organs of sense and in the brain, is based upon determinate
mental symptoms: so that psychology has long been recognised,
explicitly or implicitly, as an indispensable auxiliary of
physiological investigation. Psychologists, it is true, have been apt
to take a different attitude towards physiology. They have tended
to regard as superfluous any reference to the physical organism;
they have supposed that nothing more is required for a science of
mind than the direct apprehension of conscious processes
themselves. It is in token of dissent from any such standpoint that
the present work is entitled a "physiological psychology." We take
issue, upon this matter, with every treatment of psychology that is
based on simple self-observation or on philosophical
presuppositions. We shall, wherever the occasion seems to
demand, employ physiology in the service of psychology. We are
thus, as was indicated above, following the example of physiology
itself, which has never been in a position to disregard facts that
properly belong to psychology, - although it has often been
hampered in its use of them by the defects of the empirical or
metaphysical psychology which it has found current.

Physiological psychology is, therefore, first of all psychology. It
has in view the same principal object upon which all other forms
of psychological exposition are directed: the investigation of
conscious processes in the modes of connexion peculiar to them. It
is not a province of physiology; nor does it attempt, as has been
mistakenly asserted, to derive or explain the phenomena of the
psychical from those of the physical life. We may read this
meaning into the phrase 'physiological psychology,' just as we
might interpret the title 'microscopical anatomy' to mean a
discussion, with illustrations from anatomy, of what has been
accomplished by the microscope; but the words should be no more
misleading in the one case than they are in the other. As employed
in the present work, the adjective 'physiological' implies simply
that our psychology will avail itself to the full of the means that
modern physiology puts at its disposal for the analysis of conscious
processes. It will do this in two ways.

(I) Psychological inquiries have, up to the most recent times, been
undertaken solely in the interest of philosophy ; physiology was
enabled, by [p. 3] the character of its problems, to advance more
quickly towards the application of exact experimental methods.
Since, however, the experimental modification of the processes of
life, as practised by physiology, oftentimes effects a concomitant
change, direct or indirect, in the processes of
consciousness,--which, as we have seen, form part of vital
processes at large,--it is clear that physiology is, in the very nature
of the case, qualified to assist psychology on the side of method;
thus rendering the same help to psychology that it, itself received
from physics. In so far as physiological psychology receives
assistance from physiology in the elaboration of experimental
methods, it may be termed experimental psychology. This name
suggests, what should not be forgotten, that psychology, in
adopting the experimental methods of physiology, does not by any
means take them over as they are, and apply them without change
to a new material. The methods of experimental psychology have
been transformed--in some instances, actually remodelled--by
psychology itself, to meet the specific requirements of
psychological investigation. Psychology has adapted physiological
as physiology adapted physical methods, to its own ends.

(2) An adequate definition of life, taken in the wider sense, must
(as we said just now) cover both the vital processes of the physical
organism and the processes of consciousness. Hence, wherever we
meet with vital phenomena that present the two aspects, physical
and psychical there naturally arises a question as to the relations in
which these aspects stand to each other. So we come face to face
with a whole series of special problems, which may be
occasionally touched upon by physiology or psychology, but which
cannot receive their final solution at the hands of either, just by
reason of that division of labour to which both sciences alike stand
committed. Experimental psychology is no better able to cope with
them than is any other form of psychology, seeing that it differs
from its rivals dilly in method, and not in aim or purpose.
Physiological psychology, on the other hand, is competent to
investigate the relations that hold between the processes of the
physical and those of the mental life. And in so far as it accepts
this second problem, we may name it a psychophysics.[1] If we
free this term from any sort of metaphysical implication [p. 4] as
to the relation of mind and body, and understand by it nothing
more than an investigation of the relations that may be shown
empirically to obtain between the psychical and the physical
aspects of vital processes, it is clear at once that psychophysics
becomes for us not, what it is some times taken to be, a science
intermediate between physiology and psychology, but rather a
science that is auxiliary to both. It must, however, render service
more especially to psychology, since the relations existing between
determinate conditions of the physical organization, on the one
hand, and the processes of consciousness, on the other, are
primarily of interest to the psychologist. In its final purpose,
therefore, this psychophysical problem that we have assigned to
physiological psychology proves to be itself psychological. In
execution, it will he predominantly physiological since
psychophysics is concerned to follow up the anatomical and
physiological investigation of the bodily substrates of conscious
processes, and to subject its results to critical examination with a
view to their bearing upon our psychical life.

There are thus two problems which are suggested by the title
"physiological psychology": the problem of method, which
involves the application of experiment, and the problem of a
psychophysical supplement, which involves a knowledge of the
bodily substrates of the mental life. For psychology itself, the
former is the more essential; the second is of importance mainly
for the philosophical question of the unitariness of vital processes
at large. As an experimental science, physiological psychology
seeks to accomplish a reform in psychological investigation
comparable with the revolution brought about in the natural
sciences by the introduction of the experimental method. From one
point of view, indeed, the change wrought is still more radical: for
while in natural science it is possible under favourable conditions,
to male an accurate observation without recourse to experiment,
there is no such possibility in psychology. It is only with grave
reservations that what is called 'pure self-observation' can properly
be termed observation at all and under no circumstances can it lay
claim to accuracy. On the other hand, it is of the essence of
experiment that we can vary the conditions of an occurrence at
will and, if we are aiming at exact results; in a quantitatively
determinable way Hence, even in the domain of natural science the
aid of the experimental method becomes indispensable whenever
the problem set is the analysis of transient and impermanent
phenomena, and not merely the observation of persistent and
relatively constant objects. But conscious contents are at the
opposite pole from permanent objects; they are processes, fleeting
occurrences, in continual flux and change. In their case, therefore,
the experimental method is of cardinal importance; it and it alone
males a scientific introspection possible. For all accurate
observational implies that the object [p. 5] of observation (in this
case the psychical process) can be held fast by the attention, and
any changes that it undergoes attentively followed. And this
fixation by the attention implies, in its turn, that the observed
object is independent of the observer. Now it is obvious that the
required independence does not obtain in any attempt at a direct
self-observation, undertaken without the help of experiment. The
endeavour to observe oneself must inevitably introduce changes
into the course of mental events,--changes which could not have
occured without it, and whose usual consequence is that the very
process which was to have been observed disappears from
consciousness. The psychological experiment proceeds very
differently. In the first place, it creates external conditions that
look towards the introduction of a determinate mental process at a
given moment. In the second place, it makes the observer so far
master of the general situation, that the state of consciousness
accompanying this process remain approximately unchanged. The
great importance of the experimental method, therefore, lies not
simply in the fact that, here as in the physical realm, it enables us
arbitrarily to vary the conditions of our observations, but also and
essentially in the further fact that it makes observation itself
possible for us. The results of this observation may then be
fruitfully employed in the examination of other mental
phenomena, whose nature prevents their own direct experimental
modification.

We may add that, fortunately for the science, there are other
sources of objective psychological knowledge, which become
accessible at the very point which the experimental method fails
us. These are certain products of the common mental life, in which
we may trace the operation of determinate psychical motives: chief
among them are language, myth and custom. In part determined by
historical conditions, they are also, in part dependent upon
universal psychological laws; and the phenomena that are referable
to these laws form the subject-matter of a special psychological
discipline, ethnic psychology. The results of ethnic psychology
constitute, at the same time, our chief source of information
regarding the general psychology of the complex mental processes.
In this way, experimental psychology and ethnic psychology form
the two principal departments of scientific psychology at large.
They are supplemented by child and animal psychology, which in
conjunction with ethnic psychology attempt to resolve the
problems of psychogenesis. Workers in both these fields may, of
course, avail themselves within certain limits of the advantages of
the experimental method. But the results of experiment are here
matters of objective observation only, and the experimental
method accordingly loses the peculiar significance which it
possesses as an instrument of introspection. Finally, child
psychology and experimental psychology in the narrower sense
may be bracketed together as individual [p. 6] psychology while
animal psychology and ethnic psychology form the two halves of a
generic or comparative psychology. These distinctions within
psychology are, however, by no means to be put on a level with the
analogous divisions of the province of physiology. Child
psychology and animal psychology are of relatively slight
importance, as compared with the sciences which deal with the
corresponding physiological problems of ontogeny and phylogeny.
On the other hand, ethnic psychology must always come to the
assistance of individual psychology, when the developmental
forms of the complex mental processes are in question.

Kant once declared that psychology was incapable of ever raising
itself to the rank of an exact natural science.[2] The reasons that he
gives for this opinion have often been repeated in later times.[3] In
the first place, Kant says, psychology cannot become an exact
science because mathematics is inapplicable to the phenomenon of
the internal sense; the pure internal perception, in which mental
phenomena must be constructed,--time,--has but one dimension. In
the second place, however, it cannot even become an experimental
science, because in it the manifold of internal observation cannot
be arbitrarily varied,--still less, another thinking subject be
submitted to one's experiments, conformably to the end in view;
moreover, the very fact of observation means alteration of the
observed object. The first of these objections is erroneous; the
second is, at the least, one-sided. It is not true that the course of
inner events evinces only one dimension, time. If this were the
case, its mathematical representation would, certainly be
impossible; for such representation always requires at least two
variables, which can be subsumed under the concept of magnitude.
But, as a matter of fact, our sensations and feelings are intensive
magnitudes, which form temporal series. The course of mental
events has, therefore, at any rate two dimensions; and with this
fact is given the general possibility of its presentation in
mathematical form. Otherwise, indeed Herbart could hardly have
lighted upon the idea of applying mathematics to psychology. And
his attempt has the indisputable merit of proving once and for all
the possibility of an application of mathematical methods in the
sphere of mind.[4]

If Herbart, nevertheless, failed to accomplish the task which he set
himself, the reason of his failure is very simple; it lay in the
overweening confidence with which he regarded the method of
pure self-observation and the hypotheses whereby he filled out the
gaps that this observation leaves. It is Fechner's service to have
found and followed the true way; to have shown us how a
'mathematical psychology' may, within certain limits, be realised
in practice. Fechner's method consists in the experimental
modification of consciousness by sensory stimuli; it leads, under
favourable circumstances, to the establishment

of certain quantitative relations between the physical and the
psychical.[5]At [p. 7] the present day, experimental psychology
has ceased to regard this formulation of mental measurements as
its exclusive or even as its principal problem. Its aim is now more
general; it attempts, by arbitrary modification of consciousness, to
arrive at a causal analysis of mental processes. Fechner's
determinations are also affected, to some extent, by his conception
of Psychophysics as a specific science of the 'interactions of mind
and body.' But, in saying this, we do not lessen the magnitude of
his achievement. He was the first to show how Herbart's idea of an
'exact psychology' might be turned to practical account.

The arguments that Kant adduces in support of his second
objection, that the inner experience is inaccessible to experimental
investigation, are all derived from purely internal sources, from the
subjective flow of processes; and there, of course, we cannot
challenge its validity. Our psychical experiences are, primarily,
indeterminate magnitudes; they are incapable of exact treatment
until they have been referred to determinate units of measurement,
which in turn may be brought into constant causal relations with
other given magnitudes. But we have, in the experimental
modification of consciousness by external stimuli, a means to this
very end,--to the discovery of the units of measurement and the
relations required. Modification from without enables us to subject
our mental processes to arbitrarily determined conditions, over
which we have complete control and which we may keep constant
or vary as we will. Hence the objection urged against experimental
psychology, that it seeks to do away with introspection, which is
the sine qua non of any psychology, is based upon a
misunderstanding. The only form of introspection which
experimental psychology seeks to banish from the science is that
professing self-observation which thinks it can arrive directly,
without further assistance, at an exact characterisation of mental
facts, and which is therefore inevitably exposed to the grossest
self-deception. The aim of the experimental procedure is to
substitute for this subjective method, whose sole resource is an
inaccurate inner perception, a true and reliable introspection, and
to this end it brings consciousness under accurately adjustable
objective conditions. For the rest, here as elsewhere, we must
estimate the value of the method, in the last resort, by its results. It
is certain that the subjective method has no success to boast of; for
there is hardly a single question of fact upon which its
representatives do not hold radically divergent opinions. Whether
and how far the experimental method is in better cast, the reader
will be able to decide for himself at the conclusion of this work.
He must, however, in all justice remember that the application of
experiment to mental problems is still only a few decades old.[6]

The omission, in the above list of the various psychological
disciplines, of any mention of what is called rational psychology is
not accidental. The term was introduced into mental science by C.
Wolff (1679-1754), to denote a knowledge of the mental life
gained, in independence of experience, simply and solely [p. 8]
from metaphysical concepts. The result has proved, that any such
metaphysical treatment of psychology must, if it is to maintain its
existence, be constantly making surreptitious incursions into the
realm of experience. Wolff himself found it necessary to work out
an empirical psychology, alongside of the rational: though it must
be confessed that, in fact, the rational contains about as much
experience as the empirical and the empirical about as much
metaphysics as the rational. The whole distinction rests upon a
complete misapprehension of the scientific position, not only of
psychology, but also of philosophy. Psychology is, in reality, just
as much an experiential science as is physics or chemistry. But it
can never be the business of philosophy to usurp the place of any
special science; philosophy has its beginnings, in every case, in the
established results of the special sciences. Hence the works upon
rational psychology stand in approximately the same relation to the
actual progress of psychological science as does the
nature-philosophy of Schelling or Hegel to the development of
modern natural science.[7]

There are certain psychological works, still current at the present
time, which bear the word 'empirical' upon their title-pages, but
make it a matter of principle to confine themselves to what they
term a 'pure' introspection. They are, for the most part, curious
mixtures of rational and empirical psychology. Sometimes the
rational part is restricted to a few pages of metaphysical discussion
of the nature of mind; sometimes- as in the great majority of books
of the kind emanating from the Herbartian School --certain
hypotheses of metaphysical origin are put forward as results of
self-observation. It has been well said that if a prize were offered
to the discovery by this whole introspective school of one single
undisputed fact, it would be offered in vain.[8] Nevertheless, the
assurance of the Herbartians is incredible. Their compendia
appear, one after another; and the memory of the students who use
them is burdened with a mixed medley of purely imaginary
processes. On the other side, the supreme advantage of the
experimental method lies in the fact that it and it alone renders a
reliable introspection possible, and that it therefore increases our
ability to deal introspectively with processes not directly
accessible to modification from without. This general significance
of the experimental method is being more and more widely
recognised in current psychological investigation; and the
definition of experimental psychology has been correspondingly
extended beyond its original limits. We now understand by
'experimental psychology' not simply those portions of psychology
which are directly accessible to experimentation, but the whole of
individual psychology. For all such psychology employs the
experimental method: directly, where its direct use is possible; but
in all other cases indirectly, by availing itself of the general results
which the direct employment of the method has yielded, and of the
refinement of psychological observation which this employment
induces.

Experimental psychology itself has, it is true, now and again
suffered relapse into a metaphysical treatment of its problems. We
recognise the symptoms whenever we find 'physiological
psychology' defined, from the outset, in such a way as to give it a
determinate metaphysical implication. The task now assigned to
the science is that of the interpretation of conscious phenomena by
[p. 9] their reference to physiological conditions. Usually, the
infection spreads still farther, and the same view is taken of the
problem of psychology at large. As regards sensations, the
elements out of which they are compounded, conscious processes
(we are told) have their specific character, their peculiar
constitution; but it is impossible by psychological means to
discover uniformities of connexion among these elements. Hence
the only road to a scientific description or explanation of complex
mental experiences lies through the knowledge of the
physiological connexions obtaining among the physiological
processes with which the psychical elements are correlated.[9] On
this conception, there is no such thing as psychical, but only
psychical causation, and every causal explanation of mental
occurrence must consequently be couched in physiological terms.
It is accordingly termed the theory of 'psychophysical materialism.'
The theory as such is by no means a new thing in the history of
philosophy. All through the eighteenth century it was struggling
for mastery with the rival theory of mechanical materialism, which
explained the psychical elements themselves as confused
apprehensions of molecular motions. But it presents a novel
feature in its endeavour to press physiological psychology into the
service of the metaphysical hypothesis and thus apparently to
remove this hypothesis from the metaphysical sphere,--so that
psychological materialism becomes for its representatives
compatible even with a philosophical idealism of the order of Kant
or Fichte. Since psychology, from this point of view, forms a
supplement to physiology, and therefore takes its place among the
natural sciences, it need, as a matter of fact, pay no further regard
either to philosophy or to the mental sciences. That the mental life
itself is the problem of psychology,--this is mere dogma, handed
down to us by past ages.[10]Yet after all, the assertion that there is
no such thing as psychical causation, and that all psychical
connexions must be referred back to physical, is at the present day
this: it is an assumption which, on its negative side, comes into
conflict with a large number of actually demonstrable psychical
connexions, and, on the positive, raises a comparatively very
limited group of experiences to the rank of an universal principle.
It is, we must suppose, a realisation of the inadequacy of the
arguments offered in support of these two fundamental
implications that has led certain psychologists, who would
otherwise take the same theoretical position, to divide the problem
of psychology, and to recognise the interconnexions of mental
processes as a legitimate object of inquiry, alongside of the
investigation of their dependence upon determinate physiological
process within the brain. In the psychological portion of their
works, these writers usually adopt the theory of the 'association of
ideas,' elaborated in the English psychology of the eighteenth
century.[11] They adopt it for the good and sufficient reason that
the doctrine of association, from David Hartley (1705-1757) [p.
10] down to Herbert Spencer (1820-1904), has itself for the most
part attempted merely a physiological interpretation of the
associative processes.

The materialistic point of view in psychology can claim, at best,
only the value of an heuristic hypothesis. Its justification must,
therefore, be sought first of all in its results. But it is apparent that
the diversion of the work of psychology from its proper object, the
related manifold of conscious processes, is precisely calculated to
make the experimental method comparatively barren, so far as
concerns psychology itself. And, as a matter of fact, the books
upon physiological psychology that are written from the standpoint
of materialism confine themselves almost entirely, when they are
not borrowing from the physiology of brain and sense organs, to
the beaten track of the traditional doctrine of association. Ideas are
treated, after as before, as if they were immutable objects, that
come and go, form connexions of sequence with one another, obey
in these connexions the well-known laws of habit and practice, and
finally, when arranged in certain groups, yield the not very
startling result that they can be brought under the same logical
categories that have proved generally serviceable for the
classification of all sorts of concepts.[12]

Now physiology and psychology, as we said just now, are auxiliary
disciplines, and neither can advance without assistance from the
other. Physiology, in its analysis of the physiological functions of
the sense organs, must use the results of subjective observation of
sensations; and psychology, in its turn, needs to know the
physiological aspects of sensory function, in order rightly to
appreciate the psychological. Such instances might easily be
multiplied. Moreover, in view of the gaps in our knowledge,
physiological and psychological alike, it is inevitable that the one
science will be called upon, time and again, to do duty for the
other. Thus, all our current theories of the physical processes of
light excitation are inferences from the psychological course and
character of visual sensations; and we might very well attempt,
conversely, to explain the conditions of practice and habituation,
in the mental sphere, from the properties of nervous substance, as
shown in the changes of excitability due to the continued effect of
previous excitations. But one cannot assert, without wilfully
closing one's eyes to the actual state of affairs or taking theories
for facts, that the gaps in our knowledge which demand this sort of
extraneous filling are to be found only on the one side, the side of
psychology. In which of the two sciences our knowledge of
processes and of the interconnexion of processes is more or less
perfect or imperfect is a question that, we may safely say, hardly
admits of an answer. But however this may be, the assertion that
the mental life lacks all causal connexion, and that the real and
primary object of psychology is therefore not the mental life itself
but the physical substrate of that life,--this assertion stands
self-condemned. The effects of such teaching upon psychology
cannot but be detrimental. In the first place, it conceals the proper
object of psychological investigation behind facts and hypotheses
that are borrowed from physiology. Secondly and more especially,
it recommends the employment of the experimental methods
without the least regard to the psychological point of view, so that
for psychology as such their results are generally valueless. Hence
the gravest danger that besets the path of our science today comes
not from the speculative and empirical dogmas of the older
schools, but [p. 11] rather from this materialistic pseudo-science.
Antipsychological tendencies can hardly find clearer expression
than in the statement that the psychological interpretation of the
mental life has no relation whatever to the mental life itself, as
manifested in history and in society.

Besides this application of the term 'experimental psychology' in
the interests of psychological materialism, we find it used in still
another sense, which is widely different from that of our own
definition. It has become customary, more especially in France, to
employ the name principally, if not exclusively, for experiments
upon hypnotism and suggestion. At its best, however, this

usage narrows the definition of 'experimental psychology' in a
wholly inacceptable way. If we are to give the title of
'psychological experiment' to each and every operation upon
consciousness that brings about a change of conscious contents,
then, naturally, hypnotisation and the suggestion of ideas must be
accounted experiments. The inducing of a morphine narcosis, and
any purposed interference with the course of a dream
consciousness, would fall under the same category. But if the
principal value of the psychological experiment lies in the fact that
it makes an exact introspection possible, very few of these
modifications of consciousness can be termed true psychological
experiments. This does not mean of course, that experiments with
suggestion may not, under favourable circumstances,--in the hands
of an experimenter who is guided by correct psychological
principles, and who has at his command reliable and
introspectively trained observers,--yield results of high importance
to psychology: so much, indeed, is proved by Vogt's observations
on the analysis of the feelings in the hypnotic state.[13] But in
such cases the conditions necessary to the performance of accurate
experiments are, it is plain, peculiarly difficult of fulfilment; and
the great majority of what are called 'hypnotic experiments' either
possess, accordingly, no scientific value at all or lead to the
observation of interesting but isolated facts, whose place in the
psychological system is still uncertain.[14]

2. Survey of the Subject

Physiological psychology is primarily psychology and therefore
has for its subject the manifold of conscious processes, whether as
directly experienced by ourselves, or as inferred on the analogy of
our own experiences from objective observation. Hence the order
in which it takes up particular problems will be determined
primarily by psychological considerations; the phenomena of
consciousness fall into distinct groups, according to the points of
view from which they are successively regarded. At the same time,
any detailed treatment of the relation between the psychical and
physical aspects of vital processes presupposes a digression into
anatomy and physiology such as would naturally be out of place in
a purely psychological exposition. While, then, the following
Chapters of this work are arranged in general upon a systematic
plan, the author has not always observed the rule that the reader
should be adequately prepared, at each stage [p. 12] of the
discussion, by table contents of preceding Chapters. Its disregard
has enabled him to avoid repetition; and he has acted with the less
scruple, in view of the general understanding of psychology which
the rending of a book like the present implies. Thus a critical
review of the results of brain anatomy and brain physiology, with
reference to their value for psychology, presupposes much and
various psychological knowledge. Nevertheless, it is necessary, for
other reasons, that the anatomical and physiological considerations
should precede the properly psychological portion of the work.
And similar conditions recur, now and again, even in Chapters that
are pre-eminently psychological.[l5]

Combining in this way the demands of theory and the precepts of
practical method, we shall in what follows (1) devote a first Part to
the bodily substrate of the mental life. A wealth of new knowledge
is here placed at our disposal by the anatomy and physiology of the
central nervous system, reinforced at various points by pathology
and general biology. This mass of material calls imperatively for
examination from the psychological side: more especially since it
has become customary for the sciences concerned in its acquisition
to offer all varieties of psychological interpretation of their facts.
Nay, so far have things gone, that we actually find proposals made
for a complete reconstruction of psychology itself, upon an
anatomical and physiological basis! But, if we are seriously to
examine these conjectures and hypotheses, we must, naturally,
acquaint ourselves with the present status of the sciences in
question. Even here, however, our presentation of the facts will
depart in some measure from the beaten path. Our aim is
psychological: so that we may restrict ourselves, on the one hand,
to matters of general importance, while on the other we must lay
special emphasis upon whatever is significant for psychology.
Thus it cannot be our task to follow brain anatomy into all the
details which it has brought to light concerning the connexions of
fibres within the brain,--into all those minute points whose
interpretation is still altogether uncertain, and whose truth is often
and again called in question. It will only be necessary for us to
obtain a general view of he structure of the central organs and of
such principal connexions of these with one another and with the
peripheral organs as have been made out with sufficient certainty.
We may then in the light of reasonably secure principles of nerve
physiology and of our psychological knowledge, proceed to
discuss the probable relations of physiological structure and
function to the processes of consciousness.

(2) We shall then, in a second Part, begin our work upon the
problem [p. 13] of psychology proper, with the doctrine of the
elements of the mental life. Psychological analysis leaves us with
two such elements, of specifically different character: with
sensations, which as the ultimate and irreducible elements of ideas
we may term the objective elements of the mental life, and with
feelings, which accompany these objective elements as their
subjective complements, and are referred not to external things but
to the state of consciousness itself. In this sense, therefore, we call
blue, yellow, warm, cold, etc., sensations; pleasantness,
unpleasantness, excitement, depression, etc., feelings. It is
important that the terms he kept sharply distinct, in these assigned
meanings, and not used indiscriminately, as they often are in the
language of everyday life, and even in certain psychologies. It is
also important that they be reserved strictly for the psychical
elements, and not applied at random both to simple and to
complex contents,--a confusion that is regrettably current in
physiology. Thus in what follows we shall not speak of a manifold
of several tones or of a coloured extent as a 'sensation', but as an
'idea'; and when we come to deal with the formations resulting
from a combination of feelings we shall term them expressly
'complex feelings' or (if the special words that language offers us
are in place) 'emotions,' 'volitions,' etc. This terminological
distinction cannot of course, tell us of itself anything whatsoever
regarding the mode of origin of such complex formations from the
psychical elements. It does, however, satisfy the imperative
requirement that the results of psychological analysis of complex
conscious contents be rendered permanent, when that analysis is
completed, by fitting designations. As for these results themselves,
it need hardly be said that the mental elements are never given
directly as contents of consciousness in the uncompounded state.
We may learn here from physiology, which has long recognised
the necessity of abstracting, in its investigations of these products
of analysis, from the connexions in which they occur. Sensations
like red, yellow, warm, cold, etc., are considered by physiologists
in this their abstract character, i.e., without regard to the
connexions in which, in the concrete case, they invariably present
themselves. To employ the single term 'sensation' as well for these
ultimate and irreducible elements of our ideas as for the surfaces
and objects that we perceive about us is a confusion of thought
which works sufficient harm in physiology, and which the
psychologist must once and for all put behind him.

But there is another and a still worse terminological obscurity,
common both to physiology and to psychology, which has its
source in the confusion of conscious processes themselves with the
outcome of a inner reflection upon their objective conditions. It is
all too common to find sensations so named only when they are
directly aroused by external sensory stimuli; while the sensations
dependent upon any sort of internal condition are termed ideas, [p.
14] and the word idea itself is at the same time restricted to the
contents known as memory images. This confusion is
psychologically inexcusable. There is absolutely no reason why a
sensation -- blue, green, yellow, or what not -- should be one thing
when it is accompanied simply by an excitation in the 'visual
centre' of the cortex, and another and quite a different thing where
this excitation is itself set up by the operation of some external
stimulus. As conscious contents, blue is and remains blue, and the
idea of an object is always a thing ideated in the outside world,
whether the external stimulus or the thing outside of us be really
present or not. It is true that the memory image is, oftentimes,
weaker and more transient then the image of direct perception. But
this difference is by no means constant; we may sense in dreams,
or in the state of hallucination, as intensively as we sense under the
operation of actual sensory stimuli.[16] Such distinctions are,
therefore, survivals from the older psychology of reflection, in
which the various contents of consciousness acquired significance
only as the reflective thought of the philosopher read a meaning
into them. It was an accepted tenet of this psychology that ideas
enjoy an immaterial existence in the mind, while sensation was
regarded as something that makes its way into mind from the
outside. Now all this may be right or wrong; but, whether right or
wrong, it evidently has no bearing whatever upon the conscious
process as such.

The attitude of physiological psychology to sensations and
feelings, considered as psychical elements, is, naturally, the
attitude of psychology at large. At the same time, physiological
psychology has to face a number of problems which do not arise
for general psychology: problems that originate in the peculiar
interest which attaches to the relations sustained by these ultimate
elements of the mental life to the physical processes in the nervous
system and its appended organs. Physiology tells us, with ever
increasing conviction, that these relations, especially in the case of
sensations, are absolutely uniform; and with an improved
understanding of bodily expression, of affective symptomatology,
we are gradually coming to see that the feelings too have their
laws of correlation, no less uniform, if of an entirely different
nature. But this growth of knowledge lays all the heavier charge
upon psychology to determine the significance of the various
psychophysical relations. A pure psychology could afford, if needs
must, to pass them by, and might confine itself to a description of
the elements and of their direct interrelations. A physiological
psychology, on the other hand, is bound to regard this
psychophysical aspect of the problems of mind as one of its most
important objects of investigation.

(3) The course of our inquiry proceeds naturally from the mental
ele-[p. 15] ments to the complex psychical processes that take
shape in consciousness from the connexion of the elements. These
mental formations must be treated in order; and our third Part will
be occupied with that type of complex process to which all others
are referred as concomitant processes: with the items that arise
from the connexion of sensations. Since physiological psychology
stands committed to the experimental method, it will here pay
most regard to the sense ideas aroused by external stimuli, these
being most easily brought under experimental control. We may
accordingly designate the contents of this section a study of the
composition of sense ideas. Our conclusions will however, apply
equally well to ideas that are not aroused by external sensory
stimuli; the two classes of ideas agree in all essential characters,
and are no more to be separated than are the corresponding
sensations.

The task of physiological psychology remains the same in the
analysis of ideas that it was in the investigation of sensations: to
act as mediator between the neighbouring sciences of physiology
and psychology. At the same time, the end in view all through the
doctrine of ideas is pre-eminently psychological; the specifically
psychophysical problems, that are of such cardinal importance for
the theory of sensation, now retire modestly into the background.
Physiological psychology still takes account of the physical aspect
of the sensory functions involved, but it hardly does more in this
regard than it is bound to do in any psychological inquiry in which
it avails itself of the experimental means placed at its disposal.

(4) The doctrine of sense ideas is followed by a fourth Part,
dealing with the analysis of mental processes that, as complex
products of the interconnexion of simple feelings, stand in a
relation to the affective elements analogous to that sustained by
ideas to the sensations of which they are compounded. It must not,
of course, be understood that the two sets of formations can, in
reality, be kept altogether separate and distinct. Sensations and
feelings are, always and everywhere, complementary constituents
of our mental experiences. Hence the conscious contents that are
compounded of feelings can never occur except together with
ideational contents, and in many cases the affective elements are
as powerful to influence sensations and ideas as these are to
influence the feelings. This whole group of subjective experiences;
in which feelings are the determining factors, maybe brought
under the title of Gemuthsbewegungen und Willenshandlungun. Of
these, Gemuthsbewegungen is the wider term, since it covers
volitional as well as affective processes. Nevertheless, in view of
the peculiar importance of the phenomena of will, and of the
relation which external voluntary actions bear to other organic
movements,--a relation whose psychophysical implications
constitute it a special problem of physiological psychology,--we
retain the two words side by side in the title of our section, and
limit the meaning [p. 16] of Gemuthsbewegungen on the one hand
to the emotions, and on the other to a class of affective processes
that are frequently bound up with or pass into emotions, the
intellectual feelings.[17]

(5) Having thus investigated sense ideas, emotions and voluntary
actions, the complex processes of the mental life, we pass in a fifth
Part to the doctrine of consciousness and of the interconnexion of
mental processes. The results of the two preceding sections now
form the basis of an analysis of consciousness and of the
connexions of conscious contents. For all these conscious
connexions contain, as their proximate constituents, ideas and
emotion and consciousness itself is nothing else than a general
name for the total sum of processes and their connexions. So far as
our analysis of these connexions is experimental we shall he
chiefly concerned with the arbitrary modification of sense ideas
and of their course in consciousness. When, on the other hand, we
come to consider the interconnexions of emotions and voluntary
actions, our principal dependence will be upon the results of
analysis of the processes of consciousness at large.

In these five Parts, then, we confine ourselves to a purely empirical
examination of the facts. (6) A sixth and final Part will treat of the
origin and principles of mental development. Here we shall
endeavour to set forth, in brief, the general conclusions that may
be drawn from these facts for a comprehensive theory of the
mental life and of its relation to our physical existence. So far, we
have set conscious processes and the processes of the bodily life
over against each other, without attempting any exact definition of
either. Now at last, when our survey of their interrelations is
completed, we shall be able to ascribe a definitive meaning to the
terms physical and psychical. And this will help us towards a
solution of the well worn problem of 'the interaction of mind and
body,' a solution that shall do justice to the present status of our
physiological and psychological knowledge, and shall also meet
the requirements of a philosophical criticism of knowledge itself.
Physiological psychology thus ends with those questions with
which the philosophical psychology of an older day was wont to
begin,--the questions of the nature of the mind, and of the relation
of consciousness to an external world; and with a characterisation
of the general attitude which psychology is to take up, when it
seeks to trace the laws of the mental life as manifested in history
and in society.

3. Prepsychological Concepts[18]

The human mind is so constituted, that it cannot gather
experiences [p. 17] without at the same time supplying an
admixture of its own speculation. The first result of this naive
reflection is the system of concepts which language embodies.
Hence, in all departments of human experience, there are certain
concepts that science finds ready made, before it proceeds upon its
own proper business,--results of that primitive reflection which
have left its permanent record in the concept-system of language.
'Heat' and 'light,' e.g., are concepts from the world of external
experience, which had their immediate origin in sense-perception.
Modern physics subsumes them both under the general concept of
motion. But it would not be able to do this, if the physicist had not
been willing provisionally to accept the concepts of the common
consciousness, and to begin his inquiries with their investigation.
'Mind,' 'intellect,' 'reason,' 'understanding,' etc., are concepts of just
the same kind, concepts that existed before the advent of any
scientific psychology. The fact that the naive consciousness always
and everywhere points to internal experience as a special source of
knowledge, may, therefore, be accepted for the moment as
sufficient testimony to the rights of psychology as science. And
this acceptance implies the adoption of the concept of 'mind,' to
cover the whole field of internal experience. ' Mind,' will
accordingly be the subject, to which we attribute all the separate
facts of internal observation as predicates. The subject itself is
determined wholly and exclusively by its predicates; and the
reference of these to a common substrate must be taken as nothing
more than an expression of their reciprocal connexion. In saying
this, we are declining once and for all to read into the concept of
'mind' a meaning that the naive linguistic consciousness always
attaches to it. Mind, in popular thought, is not simply a subject in
the logical sense, but a substance, a real being; and the various
'activities of mind' as they are termed, are its modes of expression
or action. But there is here involved a metaphysical
presupposition, which psychology may possibly be led to honour at
the conclusion of her work, but which she cannot on any account
accept, untested, before she has entered upon it. Moreover, it is not
true of this assumption as it was of the discrimination of internal
experience at large, that it is necessary for the starting of the
investigation. The words coined by language to symbolise certain
groups of experiences still bear upon them marks which show that,
in their primitive meanings, their stood not merely [p. 18] for
separate modes of existence, for 'substances,' in general but
actually for personal beings. This personification of substances has
left its most indelible trace in the concept of genus. Now the
word-symbols of conceptual ideas have passed so long from hand
to hand in the service of the understanding, that they have
gradually lost all such fanciful reference. There are many cases in
which we have seen the end, not only of the personification of
substances, but even of the substantialising of concepts. But we are
not called upon, on that account, to dispense with the use whether
of the concepts themselves or of the words that designate them.
We speak of virtue, honour, reason; but our thought does not
translate any one of these concepts into a substance. They have
ceased to be metaphysical substances, and have become logical
subjects. In the same way, then, we shall consider mind, for the
time being, simply as the logical subject of internal experience.
Such a view follows directly from the mode of concept-formation
employed by language, except that it is freed of all those
accretions of crude metaphysics which invariably attach to
concepts in their making by the naive consciousness.

We must take up a precisely similar attitude to other ready-made
concepts that denote special departments or special relations of the
internal experience. Thus our language makes a distinction
between 'mind' and 'spirit.' The two concepts carry the same
meaning, hut carry it in different contexts: their correlates in the
domain of external experience are 'body' and 'matter.' The name
'matter' is applied to any object of external experience as it
presents itself directly to our senses, without reference to an inner
existence of its own. 'Body' is matter thought of with reference to
such an inner existence. 'Spirit,' in the same way, denotes the
internal existence as considered out of all connexion With an
external existence; whereas 'mind,' especially where it is explicitly
opposed to spirit, presupposes this connexion with a corporeal
existence, given in external experience.[19]

While the terms 'mind' and 'spirit' cover the whole field of internal
experience, the various 'mental faculties,' as they are called,
designate the special provinces of mind as distinguished by a
direct introspection. Language brings against us an array of
concepts like 'sensibility,' 'feeling,' 'reason,' 'understanding,'--a
classification of the processes given in internal perception against
which, bound down as we are to the use of these words, we are
practically powerless. What we call do, however, and what science
is obliged to do, is to reach an exact definition of the concepts; and
to arrange them upon a systematic plan. It is probable that the
mental faculties stood originally not merely for different parts of
the field of internal [p. 19] experience, but for as many different
beings; though the relation of these to the total being, the mind or
spirit, was not conceived of in any very definite way. But the
hypostatization of these concepts lies so far back in the remote
past, and the mythological interpretation of nature is so alien to
our modes of thought, that there is no need here to warn the reader
against a too great credulity in the matter of metaphysical
substances. Nevertheless, there is one legacy which has come
down to modern science from the mythopoeic age. All the
concepts that we mentioned just now have retained a trace of the
mythological concept of force; they are not regarded simply
as--what they really are--class-designations of certain departments
of the inner experience, but are oftentimes taken to be forces, by
whose means the various phenomena are produced. Understanding
is looked upon as the force that enables us to perceive truth;
memory as the force which stores up ideas for future use; and so
on. On the other hand, the effects of these different 'forces'
manifest themselves so irregularly that they hardly seem to be
forces in the proper sense of the word; and so the phrase 'mental
faculties' came in to remove all objections. A faculty, as its
derivation indicates, is not a force that must operate, necessarily
and immutably, but only a force that may operate. The influence of
the mythological concept of force is here as plain as it could well
be; for the prototype of the operation of force as faculty is,
obviously, to be found in human action. The original significance
of faculty is that of a being which acts. Here, therefore, in the first
formation of psychological concepts, we have the germ of that
confusion of classification with explanation which is one of the
besetting sins of empirical psychology. The general statement that
the mental faculties are class concepts, belonging to descriptive
psychology, relieves us of the necessity of discussing them and
their significance at the present stage of our inquiry. As a matter of
fact, one can quite well conceive of a natural science of the
internal experience in which sensibility, memory, reason and
understanding should be conspicuous by their absence. For the
only things that we are directly cognisant of in internal perception
are individual ideas, feelings, impulses, etc.; and the subsumption
of these individual facts under certain general concepts contributes
absolutely nothing toward their explanation.

At the present day, the uselessness of the faculty-concepts is
almost universally conceded. Again, however, there is one point in
which they still exercise a widespread influence. Not the general
class-concepts, but the individual facts that, in the old order of
things, were subsumed under them, are now regarded in many
quarters as independent phenomena, existing in isolation. On this
view there is, to be sure, no special faculty of ideation or feeling or
volition; put the individual idea, the individual affective process,
and the individual voluntary act are looked upon as inde-[p. 20]
pendent processes, connecting with one another and separating
from one another as circumstances determine. Now introspection
declares that all these professedly independent processes through
and through interconnected and interdependent. It is evident,
therefore, that their separation involves just the same translation of
the products of abstraction into real things as we have charged to
the account of the old doctrine of faculties, - only that in this case
the abstractions come a little nearer to the concrete phenomena.
An isolated idea, an idea that is separable from the processes of
feeling and volition, no more exists than does all isolated mental
force of 'understanding.' Necessary as these distinctions are, then,
we must still never forget that they are based upon
abstractions--that they do not carry with them any real separation
of objects. Objectively, we can regard the individual mental
processes only as inseparable elements of interconnected wholes.

The argument of the text may be supplemented here by some
further critical remarks upon the two parallel concepts of 'mind'
and 'spirit,' and upon the doctrine of mental faculties.

The English language distinguishes spirit from mind; is a second
substance-concept, with the differentia that it is not, as mind is,
necessarily bound up, by the mediation of the senses, with a
corporeal existence, but either stands in a merely external
connexion with body or is entirely free of bodily relations The
concept of spirit is accordingly used in a two-fold meaning. On the
one hand, it stands for the substrate of all inner experiences which
are supposed to be independent of the activity of the senses; on the
other, it denotes a being which has no part or lot at all in corporeal
existence. It is, of course, only in the former of these two meanings
that the concept of spirit comes into psychology. We can, however,
see at once that the first signification must logically pass over into
the second. If the connexion of spirit with body is merely external
and as it were accidental there is no reason why spirit should not
occur in the form of pure undivided substance.

Philosophical reflection could not leave the relation of mind and
spirit in the obscurity which had satisfied the needs of the naive
consciousness. Are mind and spirit different beings? Is mind a part
of spirit, or spirit a part of mind? The earliest philosophical
speculation shows clearly enough into what perplexity these
questions plunged its authors. On the one hand, they are forced by
the interconnexion of the inner experience to postulate a single
substance as its substrate; on the other, they can see no way to
escape a separation of the more abstract spiritual activities from
the bodily entanglements of sense-perception. Alongside of the
universal dualism of matter and spirit there remains the more
restricted antithesis of spirit and mind. And ancient philosophy
never succeeded in wholly overcoming this antithesis,--whether,
with Plato, it tries to get rid of the substantiality of mind by
regarding mind as a mixture of matter and spirit,[20] or whether,
with Aristotle, it transfers to spirit the notion that it has abstracted
from mind and so substitutes a coincident [p. 21] form of
definition for unity of substance.[21] Modern spiritualistic
philosophy has, in general, followed the path laid down by
PLATO, though it affirms more decidedly than PLATO did the
unity of substance in mind and spirit. The result is that all real
discrimination of the two concepts disappears from the scientific
vocabulary. If a difference is made, it is made in one of to ways,
Either spirit is taken as the general concept, within which the
individual mind is contained;[22] or spirit is confused with the
mental faculties, of which we shall speak presently, and retained
as a general designation for the 'higher' mental faculties or,
specifically, for intelligence or the faculty of knowledge. The
second usage is often accompanied, in the later works, by the
inclusion of feeling and desire in the common concept of
'disposition'; so that the mind as a whole divides into intellect and
disposition,[23] without any implication of a separation into
distinct substances. Sometimes, again, a mere difference of degree
is made between the two terms mind and spirit, and spirit ascribed
to man, while mind alone is assigned to the animals. Thus the
distinction becomes less and less definite, while at the same time
the concept of spirit loses its substantial character. So that, if we
are to give the worth a meaning that shall not anticipate the results
of later investigation, we can do no more than say that spirit, like
mind, is the subject of the inner experience, but that in it
abstraction is made from the relations of this subject to a corporeal
being. Mind is the subject of the inner experience as conditioned
by its connexion with an external existence; spirit is the same
subject without reference to such connexion. We shall accordingly,
speak of spirit and of spiritual phenomena only when we can
afford to neglect those moments of the inner experience which
render it dependent upon our sensuous existence, i.e., upon that
side of our existence which is accessible to external experience.
This definition leaves entirely open the question whether spirit
really is independent of sensibility. We can abstract from one or
more of the aspects of a phenomenon without denying that these
aspects are actually presented.

It has long been an object with philosophers to reduce the various
mental faculties distinguished by language--sensation, feeling,
reason, understanding, desire, imagination, memory, etc.--to
certain more general forms. As early as Plato's Timaeus we find an
indication of a tripartite division of the mind, in accordance with
the later discrimination of the three faculties of knowledge, feeling
and desire. Parallel with this threefold division runs another, into
the higher and lower faculties. The former, the immortal reason,
corresponds to knowledge; the latter, sensibility or the perishable
part of mind, embraces feeling and desire. Feeling or emotion is
here looked upon as mediating between reason and appetite, just as
the true idea mediates between sensuous appearance and
knowledge. But while sensation is expressly referred to the same
part of the mind as desire,[24] the mediating thought (dianoia) and
the emotion [p. 22] appear to stand in similar relation only to the
faculty of reason. Hence these attempts at classification give us the
impression two principles of division independently of each
other,--the one based upon observation of a fundamental
difference between the phenomena of cognition, feeling and
appetition, and the other upon the recognition or stages in the
process of knowledge; and that his not altogether successful
attempt to reduce the two to one came only as an afterthought. In
Aristotle the mind, regarded as the principle of life, divides into
nutrition, sensation, and faculty of thought, corresponding to the
inner most important stages in the succession of vital phenomena.
It is true that he occasionally introduces other mental faculties in
the course of his discussion; but it is quite clear that he considers
these three phenomena as the most general. Desire, in particular, is
subordinated to sensation.[25]

PLATO obtains his tripartite division by ranking the properties of
mind in the order of ethical value; Aristotle obtains his,
conformably with his definition of mind, from the three principal
classes of living beings. The plant mind is nutritive only; the
animal mind is nutritive and sensitive; the human mind is
nutritive, sensitive and rational. We can hardly doubt that the
classification, with its three separable faculties, was originally
suggested by the observation of the three kinds of living things in
nature. But, however different the source from which it springs, we
have only to omit the distinction of nutrition as a specific mental
faculty, and we find it coinciding outright with the Platonic
division into sensibility and reason. Hence it cannot itself, any
more than the various later attempts at classification, be regarded
as a really new system.

The most influential psychological systematist of modern times,
WOLFF, employs both of the Platonic divisions, side by side, but
makes the faculty of feeling subordinate to that of desire. The
consequent dichotomy runs through his whole system. He first of
all separates cognition and desire, and then subdivides each of
these into a lower and a higher part. The further progress is shown
in the following table.

I. FACULTY OF KNOWLEDGE

1. Lower Faculty of knowledge.--Sense, Imagination, Poetic
faculty, Memory (remembering and forgetting).

2. Higher Faculty of Knowledge.--Attention and reflection.
Understanding.

II. FACULTY OF DESIRE

1. Lower Faculty of Desire.--Pleasantness and unpleasantness,
Sensuous desire and sensuous aversion. Emotions.

2. Higher Faculty of Desire.--Volition (affirmation and
negation).Freedom.

This classification has its proximate source in the Leibnizian
distinction of ideation and appetition as the fundamental forces of
the monads. It shows a great advance upon previous systems in not
confining the faculty of feeling and desire to emotion and sensuous
desire, but giving it the same range as the faculty of knowledge, so
that the old difference in ethical value disappears. On the other
hand, it is obvious that the special faculties grouped under the four
main rubrics are not distinguished upon any systematic principle;
their arrangement is purely empirical. The classification
underwent many changes at the hands of WOLFF'S disciples. We
frequently find knowledge and feeling taken as the [p. 23] two
principle faculties, or feeling added as intermediary to knowledge
and desire. This last scheme is that adopted by KANT. WOLFF'S
thought, even in the empirical psychology, is guided by his
endeavour to reduce all the various faculties to a single
fundamental force, the faculty of ideation; and his rational
psychology is largely devoted to this task. KANT disapproved of
any such attempt to obliterate given differences in the mere effort
after unification. Nevertheless, he too allows knowledge to
encroach upon the domains of the other two mental forces, in
correlating each of them with a special faculty within the sphere of
cognition. But he maintains the original diversity of cognition,
feeling and desire. The faculty of knowledge comprehends the
other two only in the sense that it is the legislative faculty of mind
at large. It is the source both of the concepts of nature and of the
concept of freedom which contains the ground of the practical
precepts of the will. It also produces the intermediate teleological
judgments and judgments of taste. So we find KANT saying that
understanding, in the narrower sense, legislates for the faculty of
knowledge, reason for the faculty of desire, and judgment for
feeling;[26] while understanding, judgment and reason are
elsewhere bracketed together as understanding in the wider
sense.[27] On the other side, KANT accepts the distinction of a
lower and a higher faculty of knowledge,--the former embracing
sensibility and the latter understanding,--but rejects the hypothesis
that they are separated by a mere difference of degree. Sensibility
is, for him, the receptive, understanding the active side of
knowledge.[28] Hence in his great Critique he opposes sensibility
to understanding. When connected with sensibility, understanding
mediates empirical concepts; alone, it gives us pure notions.[29]

It is evident that there are three principal points to be emphasised
in the course of this whole development. The first is the distinction
of the three mental faculties; the second, the tripartite division of
the higher faculty of knowledge; and the third, the relation of this
to the three principal faculties. Th first is, in all essentials, a legacy
from the Wolffian psychology: the other two are peculiar to
KANT. Previous philosophy had, in general, defended reason
(logoV) as that activity of mind which by inference (ratiocinatio)
gives account of the grounds of things. The definition was,
however, compatible with various views of the position of reason.
Sometimes, just as in Neoplatonism, reason was subordinated to
understanding (nouV, intellectus); the latter is a source of
immediate knowledge, while the activity of inference implies
commerce with the world of sense. Sometimes, it was ranked
above understanding, as the means whereby we penetrate to the
ultimate grounds of things. Sometimes, again it was considered as
a special mode of manifestation of understanding. Illustrations of
all three views may be found in the scholastic philosophy. The
cause of this varying estimate of the place of reason is to be sought
in the fact that the term ratio was used in two distinct senses. On
the one hand, it meant the ground of a given consequence of
individual truths, the 'reason for'; on the other, the capacity of
ratiocinatio, of inferring individual truths from their grounds of
'reasoning'. First of all, ratio makes its appearance among the
mental faculties, in this latter significance, as faculty of inference;
later on, it appears [p. 24] also as a faculty of insight into the
grounds of things. And wherever the emphasis fell upon this
second meaning, reason shone forth as the very organ and
instrument of religious and moral truths, or as a purely
metaphysical faculty contradistingished from understanding,
whose concepts could never pass the bounds of outer or inner
sense-experience. A definition which includes both meanings of
'reason' makes it the faculty whereby we penetrate the
interconnexion of universal truths.[30] Now KANT set out from
the first of the three views above mentioned, the view which
regards understanding as the faculty of concepts and reason as the
faculty of inference. And he might well be encouraged to attempt,
by the help of logic, to carry out to its conclusion the division of
the higher faculty of knowledge which this view adumbrates,
seeing that he had already achieved entire success in a similar
undertaking, his deduction of the categories. He accordingly
assumed that, since judgment stands midway between concept and
inference (conclusion), the faculty of judgment stands midway
between the faculties of understanding and reason. He had
however, in his great Critique, sought to bring the two aspects of
the concept of reason into a more vital relation by his doctrine of
the unconditioned. In the conclusion, reason subsumes a judgment
under its general rule. Now it must proceed, in the same way, to
subordinate this rule to a higher condition; and so on, until in the
last resort it arrives at the idea of an unconditioned. This idea,
then, in its various forms as mind, world and God, remained the
peculiar property of reason in the narrower sense; while all
concepts and principles a priori, from which reason as faculty of
inference derives individual judgments, became the exclusive
property of understanding. So we find reason playing a curious
double part in the Kantian philosophy. As faculty of inference, it is
the handmaid of understanding, charged with the and principles
which understanding propounds. As faculty of transcendent ideas,
it ranks high above understanding. Understanding is directed
merely upon the empirical interconnexion of phenomena. If it
follow the idea of reason at all, it follows it only as a regulative
principle, which prescribes the course that shall lead to a
comprehension of phenomena into an absolute whole,--something
of which understanding itself has no conception. It is, however,
this regulative office of the ideas of reason that gives them their
practical value. For the moral law, in KANT, is not constitutive,
but regulative; it does not say how we really act, but how we ought
to act. At the same time, by the imperative form in which it
demands obedience, it proves the truth of the idea of
unconditioned freedom of the will.[31] In fine, then, reason
legislates for the faculty of desire, just as understanding legislates
for the faculty of knowledge. For feeling, which stands midway
between cognition and desire, there then remains only the faculty
of judgment, which in like manner stands midway between the
faculty of concepts and the faculty of inference.[32] The three
fundamental faculties of mind are thus referred to the three modes
of manifestation of the faculty of knowledge distinguished by
formal logic. And we see at once how largely this reference is the
product of an artificial schematisation suggested by the logical
forms. This intellectualism has also had its reactive influence upon
the treatment of the mental faculties; KANT pays attention only to
the higher expressions of his three principal faculties. Now it may
[p. 25] be doubted whether the totality of phenomena embraced by
the first faculty can properly be summed up in the word
'knowledge.' But, at all events, it is obvious that the limitation of
pleasant and unpleasant feeling to the judgment of aesthetic taste,
and the reference of the faculty of desire to the ideal of the good,
are not suited to serve as the starting-point of a psychological
consideration.

HERBART'S criticism of the faculty-theory is principally directed
against the form which it had assumed in the systems of WOLFF
and KANT. The heart of his argumentation lies in the two
following objections. (1) The mental faculties are mere
possibilities, which add nothing to the facts of the inner
experience. Only the individual facts of this experience, the
individual idea and feeling and what not, can really be predicated
of the mind. There is no sensibility before sensation, no memory
before the stock of ideas which it lays up. Hence these concepts,
notions of possibility, cannot be employed for the derivation of the
facts.[33] (2) The mental faculties are class-concepts, obtained by
a provisional abstraction from the inner experience, and then
raised to the rank of fundamental forces of the mind and used for
the explanation of our internal processes.[34] Both objections
seem to shoot beyond the mark at which they are primarily aimed;
they tell against methods of scientific explanation which have
found application in practically all the natural sciences. The forces
of physics, e.g., do not exist apart, by themselves, but only in the
phenomena which we term their effects; and the functional
capacities of physiology--nutrition, contractility, irritability, etc.
--are one and all 'empty possibilities.' Again, gravity, heat,
assimilation, reproduction, etc., are class-concepts, abstracted
from a certain number of similar phenomena, which have been
transformed on just the same analogy as the class-concepts of the
inner experience into forces or faculties, to be employed for the
explanation of the phenomena themselves. Indeed, if we term
sensation, thought, etc., 'manifestations' of mind, the proposition
that the mind possesses the 'faculties' of sensing, thinking, etc.,
seems to give direct expression to a conceptual construction which
comes naturally to us wherever an object evinces effects that must
be ascribed to causes lying within and not outside of the object.
Nor has HERBART any objection to raise against the use of the
concept of force at large. But he makes a distinction between force
and faculty. We assume the action of a force, in all cases where
We have learned to look upon a result as inevitable under given
conditions. We speak of a faculty, when the result may just as well
not occur as occur.[35]

Objection has been taken to this distinction, on the ground that it
presupposes a concept of faculty which is found only in the most
unscientific form of the psychological faculty-theory.[36]
Nevertheless, it must be conceded that the discrimination of the
terms is not without significance. With the development of modern
natural science, the concept of force has gradually assumed the
character of a concept of relation. The conditions which it implies
are always reciprocally determinant; it is on their co-operation that
the manifestation of force depends; and the removal of either side
of the conditions renders it null and void. Thus the concept of
force is correctly used when, e.g., the tendency to movement, that
has its source in the interrelations of physical bodies, is derived
from a force of gravitation, whereby these bodies determine each
the [p. 26] other's position in space. On the other hand, it is an
over-hasty generalisation to refer the phenomena of falling bodies
to a force of falling natively inherent in every physical body. If we
thus translate the conditions of a certain set of phenomena,
resident in a given object, into a force of which the object is
possessed, and ignore the external conditions of the observation,
we evidently have no criterion for deciding whether a variation in
the effects of this object depends upon a variation in intrinsic or in
extrinsic conditions. The result is confusion: disparate phenomena
are brought together, and (what is of more frequent occurrence)
related phenomena rested apart. Many of the forces distinguished
by the older physiology--the forces of procreation, of growth, of
regeneration, etc.--are, beyond all question, nothing more than
manifestations of a single force operating under different
circumstances. And the same thing is pretty generally admitted of
the final ramifications of the doctrine of mental faculties,--of the
distinction, e.g., between space-memory number-memory,
word-memory, etc. Similarly, the older physics explained the
phenomena of gravitation by appeal to a number of forces: fall by
the force of falling, the barometric vacuum by the 'horror vacui,'
the motions of the planets by invisible arms from the sun or by
vortices. But, further, the habit of abstraction from the external
conditions of phenomena may easily lead to the erroneous
conception of faculty, of a force that awaits an opportunity to
produce its effect: force becomes incarnate in a mythological
being. It would, therefore, be unjust to psychology, were we to
accuse her and her alone of this aberration. Only, she has the one
great advantage over the sciences of inorganic nature, that their
work has paved the way for her advance. In their hands, the
general concepts that belong at once to the outer and the inner
experience have been purged of the errors natural to the earlier
stages of the development of thought. And along with this
advantage goes the obligation to make us of it to the full.

HERBART not only realised the untenability of the faculty-theory;
he arrived at the positive conviction that mental processes must be
considered as unitary processes. But he sought to satisfy the
requirement of unity by raising one of the products of current
psychological abstraction above all the rest. He regarded the idea
as the real and only contents of the mind. Nay, he went so far as to
declare that the idea, when once it has arisen, is imperishable,
while all the other elements of mind--feelings, emotions,
impulses--are merely the resultants of the momentary interactions
of ideas. These opinions, as we shall see later, rest upon no better
foundation than hypothesis, and bring their author, at every point,
into conflict with an exact analysis of experience.[37] For the rest,
it is obvious that the reduction of all mental processes to processes
of ideation is a survival from the intellectualism of previous
psychological systems. Nevertheless, HERBART had taken the
right path in his endeavor to avoid that atomic conception of
mental processes which simply repeats the mistakes of the old
faculty-theory in less glaring form. Unfortunately. in escaping the
one error, he was fated to fall into another. The fault of the older
view is, not that it confuses unreality with reality, but that it
substitutes for reality the products of our own discriminative
abstraction.[38]

Footnotes
[1] The word was coined by Fechner; see his Elements der
Psychophysik, 1860, i. 8. In this passage, Fechner defines
psychophysics as an "exact science of the functional relations or
relations of dependency between body and mind, or, in more
general terms, between the bodily and mental the physical and
psychical worlds"; and his main object in the Elemente is,
accordingly, to establish the laws that govern the interaction of
mental and bodily phenomena. It is clear that we have implied
here the metaphysical assumption of a substantial difference
between body and mind; we can hardly conceive, in any other way,
of the existence of such a borderland, with facts and laws of its
own. Fechner himself, however, rejected this substantial
difference, for theoretical reasons: so that in strictness he could
hardly have raised objection to such a purely empirical
formulation of the problem of psychophysics as is given in the
text. Cf. the concluding Chapter of this work.

[2] KANT, Metaphysische Aufangsgr�nde d. Naturwissenschaft. In
S„mmtliche Werke, ed. by ROSESKRANZ, V. 310.

[3] Cf. esp. E. ZELLER, Abh. d. Berliner Akad. 1881, Phil.-hist.
Cl., Abh. iii.; Sitzungsher. of the same, 1882, 295 ff.; and my
remarks upon the question, Philos. Studien, i. 250, 463 ff.

[4] HEBERT, Psychologie als Wissenschaft neu gegr�ndet auf
Erfahrung, Metaphysik u. Mathematik. In Ges. Werke, ed. by
HARTENSTEIN, vols. v., vi.

[5] FECHNER, El. d. Psychophysik, ii. 9ff. An interesting light is
thrown upon origination of the idea of 'mental measurement' in
Fechner's mind, and also upon the inspiration that he derived from
Herbart, by the "Kurze Darstellung eines neuen Princips
mathematischer Psychologie" in his Zendavesta, 1851, ii. 373 ff.
For a detailed treatment of mental measurement, see Ch. ix. below.

[6] On the question of method in general, cf. my Beitr„ge zur
Theorie der Sinneswahrnehmungen, 1862, Einleitung: Ueber die
Methoden in der Psychologie; Logik, 2nd ed., ii. 2, 151 ff.; the
essay on the problems of experimental psychology in my Essays,
Leipzig, 1885, 127 ff.; the article Selbstbeobachtung u. innere
Wahrnehmung, in the Phils. Studien, iv. 292 ff.; and
Volkerpsychologie, i. I, 1900, Einleitung.

[7] Cf. with this the essay Philosophie u. Wissenschaft, in my
Essays, I ff.; and the article Ueber d. Eintheilung d.
Wissenschaften, in the Philos. Studien, v. I ff.

[8] F. A. LANGE, Geschichte des Materialismus, 2te Aufl., ii. 383;
History of Materialism, iii., 1892, 171.

[9] H. MUNSTERBERG. Ueber Aufgaben und Methoden der
Psychologie, in Schriften der Gesellschaft f. psychol. Forshung, i.
111 ff. Practically the same position, though with minor changes
of expression, is taken by the author in his Grundz�ge der
Psychologie, i., 1900, 382 ff.

[10] MUNSTERBERG. Grundz�ge der Psychologie, Vorwort, viii.
C. the same author's Psychology and Life, 1899. This view, of the
irrelevancy of psychology to the mental sciences, is further shared
by certain modern philosophers: see the criticism of it in my
Einleitung in die Philosophie, 1901, Section 4.

[11] Cf. e.g., T. ZIEHEN, Leitfaden der physiologischen
Psychologie, 5te Aufl. 1900, 3 ff.; Introduction to Physiological
Psychology, 1895, 3 ff.

[12] On the doctrine of association. see Part v., below. For a
general criticism of psychological materialism, cf. the articles
Ueber psychische Causalit„t and Ueber die Definition der
Psychologie, in the Philos. Studien, x. 47 ff., xii. 1 ff.

[13] O. Vogt, Die directe psychologie Experimentalmethode in
hypnotischen I[unknown character] wussteseinszust„nden. In the
Zeitschr. fur Hypnotismus, v., 1897, 7, 180 ff.

[14] For a general discussion of hypnotism, see Part v., below.

[15] In my Grundriss der Psychologie (4te Aufl. 1901; Outlines of
Psychology, 1897), in which I have attempted to give an
elementary exposition of psychology so far as possible under the
exclusive guidance of psychological principles, I have adhered
more strictly to the systematic point of view. Hence the Grundriss
may be regarded in this connexion both as a supplement and as
introduction to the present work.

[16] For a more extended discussion of these terminological
questions see Ch. vii.  Section I, below.

[17] Gem�thsbewegungen, as first used above, means "complex
affective, affective-volitional and volitional processes." There is
no exact English equivalent. See BALDWIN'S Dict. of Phil. and
Psych. ii. 1902, 680. Willenshandlungen means, of course,
voluntary actions, internal and external.--TRANSLATOR.

[18] In the first four editions of the Physiologische Psychologie,
the Introduction consists of two sections, entitled respectively
Aufgabe der physiologischen Psychologie, and Psychologische
Vorbegriffe. In the present, fifth edition, the second of these
sections is replaced by an Uebersicht des Gegenstandes. I here
reprint the section on Psychologische Vorbegriffe as it appeared in
1893. It was, in all probability, omitted mainly for reasons of
space. Cf. Preface to this fifth edition. It will, I think, be found
useful by English readers in its present form, although a good deal
of its criticism is implicit in the constructions of the final chapter
of the work. I print it only after much hesitation, and with the
express reminder to the reader that the author, for whatever reason,
has not included it in the current edition of his
book.--TRANSLATOR.

[19] The German terms for 'body' and 'matter' are Leib and K”rper;
for 'mind' and 'spirit,' Seele and Geist. See BALDWIN'S Dict. Of
Phil. and Psych., ii. 680.--TRANSLATOR.

[20] Timaeus, 35. JOWETT'S Plato, iii. 453-4.

[21] The Aristotelian definition of mind in general as 'earlier or
implicit entelechy (i.e. perfect realisation) of a natural body
possessed potentially of life,' holds also of the (logoV
nouVpoihtikoV), the spirit as independent of sensibility. Spirit is,
however, the reality of the mind itself, and so can be conceived of
as separated from the body; which is not the case with the other
parts of mind. De anima, ii. sub fin. WALLACE's trans,. 65;
HAMMOND's trans. 44 f.

[22] SO WOLFF, Psychologia rationalis, SectionSection 643 ff.

[23] Geist and Gem�th.-TRANSLATOR.

[24] Timaeus, 77. Jowett's Plato, iii. 449-50.

[25] De anima, ii. 2, 3. WALLACE'S trans,. 65-77; HAMMOND'S
trans., 48-56.

[26] Kritik d. Urtheilskraft, ROSENKRANZ' ed., iv. 14 ff.
BERRNARD'S trans., 1892, 13 ff.

[27] Anthropologie, vii. 2, 100 and 104.

[28] Anthropologie, 28.

[29] Kritik d. reinen Vernunft, ii. 31, 55. MULLER'S trans., 1896,
15, 40.

[30] WOLFF, Psychologia empirica, Section 483.

[31] Kritik d. prakt. Vernunft, viii. 106.

[32] Kritik d. Urtheilskraft, iv. 15. BERNARD'S trans., 16.

[33] HERBART, Werke, vii. 611.

[34] Werke, v. 214.

[35] Werke, vii. 601.

[36] J. B. MEYER, Kant's Psychologie, 116.

[37] Cf. ch. xix. [of the present edition].

[38] Cf. with this the essay on feeling and idea, in my Essays, 199
ff.

Part I The Bodily Substrate of the Mental Life

CHAPTER I  The Organic Evolution of Mental Function
1. The Criteria of Mind and the Range of the Mental Life
THE mental functions form a part of the phenomena of life.
Wherever we observe them, they are accompanied by the
processes of nutrition and reproduction. On the other hand, the
general phenomena of life may be manifested in cases where we
have no reason for supposing the presence of a mind. Hence the
first question that arises, in an inquiry concerning the bodily
substrate of mentality, is this: What are the characteristics that
justify our attributing mental functions to a living body, an object
in the domain of animate nature?

Here, upon the very threshold of physiological psychology, we are
confronted with unusual difficulties. The distinguishing
characteristics of mind are of a subjective sort; we know them only
from the contents of our own consciousness. But the question calls
for objective criteria, from which we shall be able to argue to the
presence of a consciousness. Now the only possible criteria of the
kind consist in certain bodily movements, which carry with them
an indication of their origin in psychical processes. But where are
we justified in referring the movements of a living creature to
conscious conditions? How uncertain the answer to this question
is, especially when metaphysical prejudice has a part to play in it,
may be seen at once by an appeal to history. Hylozoism inclines to
regard every movement, even the fall of a stone, as a mental
action; Cartesian spiritualism recognises no expression of mental
life beyond the voluntary movements of man. These are extreme
views. The first is beyond all verification; the second is correct
only upon the one point that the manifestations of our own
conscious life must always furnish the standard of reference in our
judgments of similar indications in other creatures. Hence we must
not begin our search for mental function among the lower types of
organised nature, where its modes of expression are least perfect.
It is only by working our way downwards, from man to the
animals, that we shall find the point at which mental life begins.

[p. 28] Now, there are a very large number of bodily movements,
having their source in our nervous system, that do not possess the
character of conscious actions. Not only are the normal
movements of heart, respiratory muscles, blood vessels and
intestines for the most part unaccompanied by any sort of
conscious affection; we find also that the muscles subserving
change of position at the periphery of the body often react to
stimuli in a purely mechanical and automatic way. To regard these
movement-processes as mental functions would be every whit as
arbitrary as to ascribe sensation to the falling stone. When,
however, we rule out all the movements that may possibly go on
without the participation of consciousness, there remains but one
class that bears upon it the constant and unmistakable signs of an
expression of the mental life, -- the class of external voluntary
actions. The subjective criterion of the external voluntary action,
as directly given in introspection, is that it is preceded by feelings
and ideas which we take to be the conditions of the movement.
Hence a movement that we observe objectively may also be
regarded as dependent on the will if it points to similar mental
processes as its conditions.

But the discovery of this criterion does not by any means remove
the practical difficulties of our diagnosis of mind. It is not possible
to distinguish certainly in every case between a purely mechanical
reflex -- or even, in the lowest organisms, a movement due to
external physical causes, such as the imbibition of tumescent
bodies, the change of volume from fluctuations of temperature,
etc. -- and a voluntary action. We have to note; in particular, that
while there are characters by which we can argue with absolute
confidence to the existence of a voluntary action, the absence of
these characters does not always necessarily imply the absence of
such action, still less the absence of psychical functions at large.
Hence all that our inquiry can hope to accomplish is the
determination of the lower limit at which a mental life is
demonstrably present. Whether it does not, in actual fact, begin at
a still lower level must remain a matter of speculation only.

The generally accepted objective criterion of an external voluntary
action is the reference of movement to the universal animal
impulses, the nutritive and the sexual. It is only as a result of
sensory excitations that these impulses can lead the animal to a
change of place that shows the marks of a voluntary action; and
the special character that prompts us to refer such sensorily
stimulated movements to a process in consciousness is their
variability. They do not appear with mechanical regularity in
response to a given external stimulus, but are varied to suit varying
conditions, and brought into connexion with sense-impressions
previously secured. Judgment on the ground of these criteria may,
in the individual case, remain doubtful; since all vital processes,
even those that are entirely [p. 29] automatic and unconscious,
evince a certain adaptation to ends, and a certain consequence in
their successive stages. But sustained and attentive observation of
living creatures will as a general rule, enable us to decide with
certainty whether any particular manifestation of life is intelligible
only from that continuity of internal states which we name
consciousness, or whether it may possibly have arisen in the
absence of mind. That consciousness, in this sense, is an universal
possession of living organisms, from man down to the protozoa, is
beyond the reach of doubt. At the lowest levels of this
developmental series the processes of consciousness are, of course,
confined within extremely narrow limits, and the will is
determined by the universal organic impulses only in the very
simplest way. Nevertheless, the manifestations of life, even among
the lowest protozoa, are explicable only upon the hypothesis that
they possess a mind. Thus the amoeba, which is to be regarded
morphologically as a naked cell (see Fig. 2, p. 33) will sometimes
return after a short interval to the starch grains that it has come
upon in the course of its wanderings, and will incept a new portion
as nutritive material in the soft protoplasm of its body.[1] Many of
the ciliated infusoria pursue others, which they kill  and devour.[2]
These are all phenomena that point towards continuity of mental
processes, though in all probability to a continuity that extends
only over a very short space of time. They point also, at all events
in the case of the Ciliata, to a variation in the choice of means, for
the satisfaction of the organic impulses, that would be
unintelligible as a merely mechanical result of external influences.

We enter, of course, upon much less certain ground when we ask,
further, whether the mental life really makes its first appearance at
that point upon the scale of organised existence at which we notice
the external voluntary action, or whether its beginnings do not
reach back to a still lower level of life. Wherever living
protoplasm occurs, it possesses the property of contractility.
Contractile movements arise, sometimes at the instigation of
external stimuli but sometimes also in the absence of any apparent
external influence. They resemble the voluntary actions of the
lowest protozoa, and are not explicable in terms of external
physical affection, but only as the results of forces resident in the
contractile substance itself.[p. 30] They cease at once with the
cessation of life. We find them evinced both by the protoplasmic
contents of young plant-cells and by the free protoplasm occurring
throughout the animal and vegetable kingdoms. Indeed, it is
probably that all elementary organisms, whether they enjoy an
independent existence or form part of a compound organism,
possess the property of contractility at least during a certain period
of their development. Consider, e.g., the lymph corpuscles, which
are found in the blood and lymph of animals, and in pus, and
which occur as migratory elements in the tissues. They are not
only entirely similar in bodily 

configuration to certain of the lowest protozoa, but they also
undergo changes of form which, in outward appearance, are
indistinguishable from the movements of these unicellular
organisms (Fig. 1). Only, the voluntary character of these
movements is beyond the reach of demonstration. It is true that
similar structures --  particularly the colourless blood-corpuscles of
invertebrates -- have been seen to take up solid substances, and
that this action may be interpreted as an inception of food.[3] It is
true, also, that movements in response to stimulus accompany the
exercise of the digestive functions in certain plants. But in neither
case is there any definite indication of a true impulse, i.e. an
impulse determined by sensation, toward the food-stuff, or of any
sort of psychological middle term between stimulus and
movement.[4] The same thing holds of the movements of the
lower forms of algae, fungi and swarm-spores, produced by a
variable distribution of water and carbon dioxide, or by different
kinds of light rays. On the other hand, the movements of certain
bacteria are so suddenly affected by light ad by the gases of
respiration, that they at once suggest an origin in sensations, But,
here again, we cannot he sure that the changes are not simply
physical effects, as is undoubtedly the case with the movements
evolved by hygrometric changes in the environment.[5][p. 31]

We must, however, always remember, in passing judgment upon
this whole group of observations, that the demonstration of
physical conditions, to which the phenomena of protoplasmic
contraction and of the movement of elementary organisms may be
referred, is by no means incompatible with the hypothesis of
concomitant psychical processes. Physiology seeks to derive the
processes in our own nervous system from general physical forces,
without considering whether these processes are or are not
accompanied by processes of consciousness. We are bidden to
believe, both by theory of knowledge and by the philosophy of
nature, that all manifestations of life, on the physical side, are
referable to natural laws of universal validity. And physiology,
acting in accordance with this requirement, has found it justified in
every instance in which she has succeeded in reaching a solution
of her problems. It follows, then, that the existence of mental
functions can never be inferred from the physical nature of organic
movements, but only from certain special conditions attending
their performance. On the other hand, observation shows that the
chemical and physiological properties of living protoplasm are
essentially the same, whether we can prove that it manifests a
mental life or whether we cannot. This holds, in particular, of the
attributes of contractility and irritability. In physical regard,
therefore, protoplasm maintains its identity throughout. If we add
to this the fact that it is impossible to draw a hard and fast line at
the point where protoplasmic movements first begin to take on a
psychological character, -- that there is a gradual transition from
the walled-in protoplasm of the plant-cell on through the migratory
lymph-corpuscles of animals and the free-living monera and
rhizopods, to the more motile ciliated and mouth-bearing infusoria
-- we cannot resist the conjecture that psychical life and the
capacity of giving expression to it are universally represented in
contractile substance.

From the standpoint of observation, then, we must regard it as a
highly probable hypothesis that the beginnings of the mental life
date from as far back as the beginnings of life at large. The
question of the origin of mental development thus resolves itself
into the question of the origin of life. Further, if physiology is
obliged, by the uniformity of interaction of physical forces
throughout the universe, to accept the postulate that the processes
of life have their ultimate basis in the general properties of matter,
psychology finds it no less obligatory to assume, in this same
matter, the universal substrate of natural phenomena, the presence
of conditions which attain to expression as the psychical aspect of
vital phenomena. But this latter statement must not mislead us.
The latent life of inorganic matter must not be confused, as
hylozoism confuses it, with real life and actual conscious-[p. 32]
ness; nor must it be considered, with materialism, as a function of
matter. The former interpretation is wrong, because it assumes the
existence of vital phenomena at a point where not these
phenomena themselves are given, but only the common ground
upon which they rest and whereby they become possible; the
second is wrong, because it posits a one-sided dependence, where
in reality we find an interrelation of simultaneously presented but
incommensurable processes. We employ the concept of material
substance to denote the ground of all objective phenomena. Hence
it is the office of this concept to make intelligible all the various
form of physical occurrence, including the physical manifestations
of life. Now among these manifestations we find movements
which indicate the presence of a consciousness. Our postulates
concerning matter will then, explain the physical causation of such
movements, but can never account for the concomitant psychical
functions. To explain these, we must make appeal to our own
consciousness.

We cannot, of course, here at the very outset of our psychology,
return any final answer to the question of the ultimate objective
criteria of the mental life. All that we can do, at the present stage,
is to indicate in brief the position to be taken up in psychological
practice. It is however, easy to see that the wide divergence of
opinion on the subject is mainly due to the intermixture of science
with philosophy, or to a fixity of judgment that has its source in
philosophical theory. Only in this way can we account for the fact
that there may still be found, in works upon the scope of the
mental life, views that range between the two extremes current in
DESCARTES' day. One author will assert that the animals, if not
without exception, at least as far up the scale as the higher
invertebrates and the lower vertebrates, are mere reflex
machines;[6] another looks upon life and mind as convertible
terms, and accordingly endows plants as well as animals with
consciousness.[7] The former view is evidently influenced, to
some extent, by the idea that psychical and physical are
antithetical terms. The alternative (physical or psychical) is often
presented as if the one concept necessarily excluded the other, --
as, indeed it did, in the metaphysical dualism of DESCARTES.
But this is misleading. The close interconnection of the
phenomena of the physical life and the processes of consciousness
makes the relation 'physical and psychical' on the face of it, much
more probable. We should, as a matter of fact, admit at once that,
e.g., a sensation is a psychical quality, without meaning to deny
that it is accompanied by a physical process in the sense-organ and
the sense-centre. And such a coexistence of the two kinds of vital
processes is, in many crises, beyond all dispute. How far it
extends, over the phenomena of life at large, is again a question
that, naturally, cannot be answered at the outset of our
psychological investigations. But, at all events, we should be
merely obscuring the facts, if we made our first approach to them
with the alternative 'physical or psychical' in our hands. And the
danger of misinter- [p. 33] pretation is, at best, grave enough.
Many movements, that may in all probability be regarded as purely
automatic, are, as we said above, purposive in character; and many
of them, again, are self-regulating. It is, therefore, very difficult to
draw the line of division in the concrete case.[8]

We may say, then, that the mechanistic explanation of the
movements of the lower animals is not the outcome of impartial
and unprejudiced observation. But the rival theory, which ascribes
mind and consciousness to the plant-world, is in no better case.
Fechner, the chief representative of this theory, himself expressly
declares that be derived it from considerations of general
philosophy: he further attributes consciousness to the earth and the
other heavenly bodies, making this cosmic consciousness the
whole, of which the individual forms of consciousness in plant and
animal are parts.[9] Hypotheses of this sort have, no doubt, a
certain justification. They emphasise the intrinsic impossibility of
the view that mental life may suddenly appear, at some point of
time and space, as a new thing; that we need not seek for its
general conditions in the universal substrate of the vital processes.
When, however, we ask how we should conceive of these
conditions, we raise a metaphysical question, -- a question that lies
well beyond the reach of psychology and its empirical problems.

Section 2. The Differentiation of Mental Functions and of their
Physical Substrate
The organic cell in the earliest stages of its development, consists
either of a naked mass of protoplasm, contractile throughout its
substance, or of a denser and immotile cortex within which motile
protoplasm is contained. 

And the same two forms are evinced by the lowest independent
organisms in which we can observe movement-processes
indicative of psychical conditions (Fig. 2). The substrate of the
elementary mental functions is here entirely homogeneous, and
coextensive with the whole mass of the [p. 34] body. The only
sense that is plainly functioning is the sense of touch. An
impression made upon any portion of the contractile protoplasm
first of all releases a movement at the place of direct impact,
which may then extend to purposively co-ordinated motion of the
entire body. The beginnings of a differentiation of mental function
can, however, be found even in the protozoa, wherever the cortical
layer surrounding be contractile body-substance has developed
special organs of movement, cilia and flagella (Fig. 3). Oftentimes
this development goes hand in hand with a differentiation of the
nutritive functions, An [sic] oral aperture and digestive cavity are
found, and in many instances a system of open canals appears,
whose fluid contents are kept in motion by a contractile vessicle.
The cilia with which these infusoria are furnished render them
incomparably more motile than the organisms lying at the very
lowest point of the organic scale, the monera and rhizopods, which
consist merely of a viscous body-mass. They are, however, more
than organs of locomotion; they function as organs of touch, and
sometimes appear to be sensitive to light as well. The spot of red
pigment noticed in many of the infusoria may also have some
connexion with light-sensation; but we have as yet no certain
ground for regarding it as a primitive organ of vision.

In the compound organisms we observe a more radical
differentiation of mental function and its bodily substrate. The
metazoan germ-cell divides into a number of cells. These seem to
be originally of the same kind, so that not infrequently all like
manifest the primitive contractility of protoplasm in course of
time, however, they become modified in matter and form; the
tissues of the plant and animal body are derived from them and
from the products of their growth, and the structural changes are
accompanied by a more and more complete specialisation of
function. The conditions which govern this process of
differentiation, to which the whole of organic nature is subject, are
still wrapped in obscurity. Our knowledge halts abruptly at the
changes of outward form in which the internal development finds
its expression.

In the plant-world we see the nutritive functions attain such a
degree of elaboration that the organism (and this is true more
especially of the higher plants) has, so to say, , no other concern
than to increase its present stock of organic substance. In the
animal world, on the other hand, the process of evolution is
characterised by the progressive discrimination of the animal and
vegetative functions, and a consequent differentiation of these two
great provinces into their separate departments. The cell-mass of
the yolk, originally homogeneous, divides up first of all into a
peripheral and a central layer of different structural character
(Figg. 4 and 5); while the cleavage cavity gradually widens out to
form the future body-cavity.[10][p. 35] At this stage, sensation and
movement appear to reside exclusively in the outer cell-layer, the
ectoderm, while the nutritive functions are discharged by the inner
layer or entoderm. At a higher level of evolution a third

layer of cells, the mesoderm, forms between the two. The initial
stages of development are thus identical over the whole series of
forms from coelenterates to vertebrates, the differentiation of
organs beginning always with the distinction of three germinal
layers. The outermost layer is the source of the nervous system and
sense-organs, as well as of the muscular system; the innermost
furnishes the organs of nutrition; and the intermediate layer, the
vascular system. In the vertebrates, the skeleton is also derived
from the ectoderm.[11]

This discrimination of organs is accompanied by a differentiation
of the elementary constituents of the tissues. When the separation
of ectoderm and entoderm is first accomplished, the cells of the
former discharge the combined function of sensation and
movement. The initial step toward a separation of these two
cardinal functions is apparently taken in the hydridae and
medusae, where the ectoderm cells send out contractile processes
into the interior of the body. The sensory and motor functions are
here still united in a single cell, but are distributed over different
portions of it (Fig. 6).[12] In the next stage, the properties of
sensation and contractility pass to special and spatially separated
cells, while connective elements [p.36] develop, to mediate the
functional interconnexion of the different structures. There thus
arises a third class of cells, lying in the paths of connexion
between sensory and muscular cells, and acting probably as organs
for the reception and transmission of stimuli. The sensory cells
now become external organs, devoted to the reception of physical
stimuli. At the same time, they undergo a differentiation, which
fits them for excitation by various forms of movement-process in
the outside world. Similarly, the contractile cells become organs
for receiving and converting into external movements the
excitations transmitted to them. But the psychical functions par
excellence are discharged by the cells of the third class, the
nerve-cells, which are connected by their processes with both the
sensory and the muscular cells, and, as we have said, mediate the
functional interconnexion of the two group's of organs. Hence the
simplest scheme of a nervous system is given with a centrally
situated nerve-cell connected on the one hand with a sense-cell
and on the other hand with a contractile muscle-cell both directed
towards the external world, but mediating the one the reception of
sense-stimuli and the other the motor reaction upon them.

It is, however, quite certain that this simplest scheme never
actually occurs. As soon as special nerve-cells are formed at all
they are formed in numbers, joined together in longitudinal and
transverse series, so that a great many of them are connected only
by way of others of their kind with the peripheral structures. This
multiplication of the central elements means, of course, that the
process of differentiation extends to the nerve-cells themselves.
They assume various functions, according to the connexions in
which they stand with one another and with the peripheral organs.
Those lying in the neighbourhood of the terminal organs are
employed in functions, auxiliary to the strictly psycho-physical
processes, which run their after course without the participation of
consciousness. Others enter into intimate relation with the
mechanisms of nutrition; they sustain and regulate the
physiological processes of secretion and circulation. They thus
lose their place among the immediate bodily conditions of the
mental [p. 37] life, and exert only an indirect influence upon mind.
This progressive differentiation of functions and of their substrate
within the nervous system finds its expression in the relative
increase of the mass of the nervous elements, and in the
elaboration of special nerve-centres, compact bodies of nerve-cells
and their fibrillar processes. We have an instance of such centres
in the ganglia of the invertebrates, which appear at the most
various stages of development, from the comparatively simple
nerve-rings of the coelenterates and the lower worms and
molluscs, up to the brain-like ganglionic masses of the anthropods
and higher molluscs (Fig. 7).

Finally, among the vertebrates, the importance of the nerve-centres
for the whole organisation of the animal is shown, from the first,
in their relation to the external bodily form and to the development
of the various systems of organs.  

Immediately after the separation of the formative materials into
the two layers of the germ-primule, there appears in the ectoderm a
groove, open above, at the bottom of which is a streak of darker
tissue. This is the primitive streak, whose direction corresponds
with the future longitudinal axis of the embryo (Fig. 8). Presently,
the groove closes and becomes the neural tube, the primule of the
myel (spinal cord) and its sheaths.[13] The anterior portion of this;
tube gives rise, by expansion, to the primule of the brain.
Concomitantly with the closure of the neural tube begins the
differentiation of the germinating cells into nerve-cells. They
increase in size, and send out runners, which become transformed
into the various cell-processes (Fig. 9).[14]

At this point there begins a serial differentiation of function and its
physical substrate, whose investigation will form the subject of the
following Chapters. We shall set out with a consideration of the
structural elements of the nervous system in their morphological
and chemical characters. We shall next raise the question of the
nature of the processes at work within those elements; in other
words, we shall attack the problem of a physiological mechanics
of nervous substance. This discussion will be followed by a brief
description of the structural development of the nervous centres,
with especial reference to the morphology of the human brain. We
shall then be prepared to approach the two main problems that are
presented by the co-ordination of functions in the nervous system.
The first of these is the determination of the course of the paths of
nervous conduction, as conditioned by the individual connexions
of the nervous elements; and the second is the problem of the
physiological functions of the central parts, -- the last and most
important question for the relation of nervous process to the
processes of the psychical life.

Notes
[1] ROMANES, Animal Intelligence, 4th ed., 1886, 18 ff.; Mental
Evolution in Animals, 1885, 18, 55; MAX VERWORN,
Psychophysiologische Protisten-Studien,1889, 146 ff.
VERWORN'S statement that voluntary actions appear for the first
time in the Ciliata, and that all movements made in response to
stimulus by the non-ciliated protozoa, so far as they are not of
purely mechanical or chemical origin, should be interpreted as
reflexes, is evidently a result not of observation, but rather of a
forgone theoretical conviction that voluntary actions must have
developed out of reflexes. On this theory see Part iv. below.

[2] FAMINZYN, The Mental Life of the Simplest Organisms,
1890 (Russian). Quoted by BECHTEREW, Bewusstsein und
Hirnlocalisation, 1898, 6.

[3] M. SCHULTZE, Das Protoplasma der Rhizopoden, 1863.
ENGELMANN,  Beitr„ge zur Physiologie des Protoplasmas, ii.,
1869. VERWORN, Die Bewegung der lebendigen Substanz, 1892,
51 ff.; Allgemeine Physiologie, 1901, 363 ff. (General Physiology,
1899, 146 ff., 527)

[4] DARWIN, Insectivorous Plants, 1875, esp. ch. x. PFEFFER,
Pflanzenphysiologie, 2te Aufl., 1897, 364 ff.

[5] T. W. ENGELMANN, in PFLšGER'S Archiv. f. d. ges.
Physiologie, xxvi. 537; xxix. 415; xxx. 95. PFEFFER,
Untersuchungen aus d. botan. Institut zu T�bingen, i. 363, 483; ii.
582. For further details, see Ch. vii. Section 3, below. On the
physical causes of proto-plasmic movement, cf. BšTSCHLI,
Untersuchungen �ber mikroskopische Sch„ume und das
Protoplasma. 1892, 172.

[6] A. BETHE, D�rfen wir den Ameisen und Bienen psychische
Qualit„ten zuschreiben? In PFLšGER'S Arch. f. d. ges. Physiol.,
1xx. 1898, 15 ff. Cf. the critical remarks of WASSMANN, Die
psychischen F„higkeiten der Ameisen, 1899, and Biol.
Centralblatt, xviii. 1898, 578.

[7] FECHNER, Nanna oder �ber das Seelenleben der Pflanzen,
1848; 2nd ed., 1899.

[8] Cf. with this the later discussions of impulsive movement (Part
iv.) and of consciousness (Part v.).

[9] FECHNER, Zendavesta oder �ber die Dinge des Himmels und
des Jenseits, i., 851, 2nd ed., 1901.

[10] The relations of the various cavities, three or four in number,
are in reality much more complicated. It would be more nearly true
to say that, where the change indicated in the text takes place, the
body-cavity gradually replaces the cleavage-cavity. Cf. MINOT,
Embryology, 1897, ch. ix. -- TRANSLATOR.

[11] The author gives no references here. The mesoderm is now
divided, by the best writers, into mesothlium, the source of the
muscles and mesodermic glands, and mesenchyma, the source of
connective and skeletal tissue. The derivation of the mesenchyma
itself is still an open question. -- TRANSLATOR.

[12] KLEINENBERG, Hydra, eine
anatomisch-entwicklungsgeschichtliche Untersuchung 1872, 21 ff.
O and R. HERTWIG,  Das Nervensystem und die Sinnesorgane
der Medusen, 1878, 157. [The cells from the epithelial layer of
Hydra shown in Fig. 6 (KLEINENBERG'S 'neuromuscular' cells)
are now to be regarded as muscle-cells. Later Note, by AUTHOR.]

[13] The myelic furrow is now known to be entirely distinct from
the primitive groove. See O. HERTWIG, Embryology, 79 ff., 125,
416 ff. -- TRANSLATOR.

[14] HIS, Archiv f�r Anatomie u. Physiologie, Anat. Abth. 1890, 
95.

CHAPTER II Structural Elements of the Nervous System 

Section 1. Morphological Elements

THE nervous system is made up of three kinds of morphological
elements: (1) cells of peculiar form and structure, the nerve-cells
or ganglion cells; (2) fibrous structures, originating as outgrowths
from the cells, -- the nerve-fibres; and (3) a ground-reticulum,
which in places is finely granular and in places fibrillar, and which
consists of the terminal ramifications of the nerve-fibres and
processes of the nerve-cells. To these must be added (4) a
sustentacular substance, fibrous or amorphous in structure, which
is regarded as a form of connective tissue.[1] The nerve-cells, with
the fibrillar ground-reticulum that surrounds them, are essential
constituents of all the central parts. In the higher nervous centres,
however, they are restricted to definite areas, which, partly from
their rich supply of capillary blood-vessels and partly from the
presence of pigment-granules, collected both in the protoplasm of
the cell-bodies and in the ground-reticulum, possess a darker
coloration than the surrounding tissue. This grey substance
contrasts so sharply with the white or myelinic substance that the
distribution of cell-groups through the central organs may readily
be followed by the naked eye. The myelinic substance itself owes
its peculiar character mainly to the myelinic sheaths which enclose
the nerve-fibres issuing from the grey substance. The connective
tissue cement-substance occurs in three principal forms. As a soft
and for the most part amorphous mass, the neuroglia, it serves to
support the central nerves and cells. In the form of endoneurium
and perineurium,[2] a denser tissue, showing tendon-like
fibrillation, it extends among and surrounds the peripheral nerves.
As the primitive sheath of Schwann, a membrane of glassy
transparency and great elasticity, nucleated at intervals, it encases
nearly all peripheral and a portion of the central nerve-fibres.
These cement-substances form a sustentacular framework for the
nervous elements. They serve, further, to carry the blood-vessels.
And the perineurium [2][p. 40] imparts to the peripheral nerves,
which have no solid wall of bone to protect them, the necessary
power of resistance to mechanical injury.

(a) -- The Nerve-Cells
It is probable that the nerve-cells (Figg. 10-14) are everywhere
devoid of a true cell-cortex. They vary in form from spherical to
irregularly angular, and differ so extraordinarily in size that some
can hardly be distinguished with certainty from the minute
corpuscles of the connective tissue, while others are visible to the
naked eye. A clear nucleus, plainly vesicular in form, and provided
with a large nucleolus, stands out in sharp contrast to the dully
pigmented protoplasm. In the central organs the cells are
embedded directly in the soft substance of the supporting tissue; in
the ganglia, they are usually surrounded with an elastic sheath of
connective tissue, often directly continuous with the primitive
sheath of a nerve-fibre proceeding from them. The nerve-cells are
characterised by their processes, one of which usually passes over
directly into a nerve fibre, while the others ramify, if not
immediately, after running a brief course, into fine fibrils. The
former is called the axis-cylinder, nerve-[p. 41] process or neurite;
the latter are termed protoplasmic processes or dendrites.
Secondary dendritic processes may also arise, not from the cell
itself, but from its neurite (Fig. 14, c). They are then named
collaterals. The two types of process are shown with special
clearness in many of the larger cells of the myel (spinal cord) and
brain of vertebrates.

The nerve-fibres do not form independent elements of the nervous
system. They originate, as embryology teaches us (Fig. 9), in
outgrowths from nerve-cells, and they remain throughout in
connexion with the cells whose processes they are. We may
accordingly consider the nervous system in its entirety as a vast
conglomerate of nerve-cells, all woven together by fibrillar
runners. Under these conditions, the only processes of the central
cells that attain to any measure of apparent independence, as
fibrillar elements, are those entering into connexion with the
peripheral organs. But even the fibrils of the muscular and
cutaneous nerves, which in many cases extend without break over
large distances, are really nothing more than cell processes long
drawn out. It is, therefore, the nerve-cell that is the main variable
in the nervous system. Both in number and nature of its processes
and in its own internal structure, the cell evinces characteristic
differences, often strongly marked, from one part of the nervous
system to another.[3] When highly magnified, most nerve-cells
show, even without treatment by selective reagents, a fibrillated
structure; clusters of granules are set, in scattered masses, between
the meshes of this fibrillar network, and a special network of
granules and fibrillae encloses the nucleus (Fig. 10). The granular
deposits are named, from their discoverer, the corpuscles of Nissl;
they are also known as tigroid bodies, or as chromophilous
substance. Colour-staining brings them out with greater clearness,
since they have an affinity for the dyes of the histologist, while the
fibrillae and the amorphous ground-substance remain unaffected
(Fig. 11). It appears, further, that these bodies stand in a peculiar
relation to the different forms of cell-process; they are assembled
in greater numbers at the points of origin of the dendrites, but are
entirely absent from the part of the cell that gives off the neurite or
axis-cylinder (Fig. 12, lower right-hand portion). Finally, besides
this network of fibrillae which run their course within the
substance of the cell, and whose continuity with the cell-processes
evidences their nervous character, there is sometimes found a
pericellular reticulum, which, basket-like, encloses the whole outer
wall of the cell. Its fibrillae can, in most cases, be traced  [p. 42]
into the dendrites, so that they too are, in all probability, to be
looked upon as nervous structures (Fig. 13).

Nerve-cells are classified, according to the number of processes
they send out, as unipolar, bipolar and multipolar. 

Unipolar cells are, however, always of rare occurrence; and, where
they occur, have probably arisen secondarily, in course of growth
from the originally bipolar form, by a fusion of its two processes,
-- which, we may note, divide again immediately after their
emergence from the cell (see Fig. 21, z, p. 50). The bipolar cell is
found more especially in the peripheral regions, e.g. in the spinal
ganglia, in the retina, and (to some extent) in the ganglia of the
sympathetic system. The great majority of nerve-cells are,
however, multipolar. As a rule, every such cell gives off a single
neurite, and an indeterminate number of dendrites. The divergent
characters not only of the processes themselves, but also of the
portions of the cell with which they are connected (Fig. 12) render
it, in the present case, an exceedingly probable hypothesis, that the
difference of structure is paralleled by a corresponding difference
of function. As a matter of fact; the fibrils of the large cells of the
ventral cornua of the myel that pass over into the motor nerves, are
without exception neuritic; while the processes that tend from the
same cells towards the higher regions of the myel are dendritic in
nature. RAMON Y CAJAL has accordingly suggested that the
dendrites are devoted exclusively to cellipetal, the neurites to
cellifugal conduction.[4] This scheme can, however, hardly he
applied to all nerve-cells, without exception, since there are many
cases in which no clear difference between the various
cell-processes can be made out.

For the rest, over and above their different manner of origination
from the cell body, their shorter course, and their greater wealth of
branches, the dendrites are morphologically distinguishable from
the neurites by their character as 'protoplasmic' processes; their
irregular [p. 43] nodosity (Fig. 14) suggests the pseudopodial
processes of the Rhizopoda (Fig. 2). They have also been observed,
under the action of mechanical, chemical or electrical stimulation,
to make amoeboid movements; though it  is doubtful whether
these changes are to be interpreted as vital phenomena, on the
analogy of the contraction of protoplasm and of muscular tissue, or
whether they are not rather simply the direct physical and chemical
effects of the stimuli applied.[5]

These differences between the two kinds of cell-processes are,
however, as we said above, not equally well marked in all cases. In
particular, the difference in length and character of course may be
comparatively slight, or may even disappear altogether, the
neurite, like the dendrite, dividing after a brief period into a large
number of delicate branches. It is also not uncommon to find cells,
especially cells of small size, whose processes show no distinct
sign of difference, of whatever sort. The cells [p. 44] with
processes of markedly different form are usually termed, from
their discoverer, the cells of DEITERS (Fig. 12); cells with quickly
dividing neurites are known as cells of GOLGI'S type; and the cells
without marked distinction of the processes are called
intermediary or intercalary cells.[6]



Finally, the dendrites, like the neurite, evince certain structural
differences. Sometimes, as in the pyramidal cells of the cerebral
cortex (Fig. 14) they divide without much complication, their
branches tending in definite directions. Sometimes, again, as in the
large PURKINJE cells of the cerebellar cortex (Fig. 15), their
ramifications are exceedingly complex and widely extended.

(b) -- The Nerve-Fibres

We have seen that the nerve-process issuing from