The dualism of mind and matter, if we have been right so far, cannot be
allowed as metaphysically valid. Nevertheless, we seem to find a certain
dualism, perhaps not ultimate, within the world as we observe it. The
dualism is not primarily as to the stuff of the world, but as to causal
laws. On this subject we may again quote William James. He points out
that when, as we say, we merely "imagine" things, there are no such
effects as would ensue if the things were what we call "real." He takes
the case of imagining a fire.
"I make for myself an experience of blazing fire; I place it near my
body; but it does not warm me in the least. I lay a stick upon it and
the stick either burns or remains green, as I please. I call up water,
and pour it on the fire, and absolutely no difference ensues. I account
for all such facts by calling this whole train of experiences unreal, a
mental train. Mental fire is what won't burn real sticks; mental water
is what won't necessarily (though of course it may) put out even a
mental fire.... With 'real' objects, on the contrary, consequences
always accrue; and thus the real experiences get sifted from the mental
ones, the things from our thoughts of them, fanciful or true, and
precipitated together as the stable part of the whole experience--chaos,
under the name of the physical world."*
* "Essays in Radical Empiricism," pp. 32-3.
In this passage James speaks, by mere inadvertence, as though the
phenomena which he is describing as "mental" had NO effects. This is, of
course, not the case: they have their effects, just as much as physical
phenomena do, but their effects follow different laws. For example,
dreams, as Freud has shown, are just as much subject to laws as are the
motions of the planets. But the laws are different: in a dream you may
be transported from one place to another in a moment, or one person
may turn into another under your eyes. Such differences compel you to
distinguish the world of dreams from the physical world.
If the two sorts of causal laws could be sharply distinguished, we could
call an occurrence "physical" when it obeys causal laws appropriate to
the physical world, and "mental" when it obeys causal laws appropriate
to the mental world. Since the mental world and the physical world
interact, there would be a boundary between the two: there would be
events which would have physical causes and mental effects, while there
would be others which would have mental causes and physical effects.
Those that have physical causes and mental effects we should define as
"sensations." Those that have mental causes and physical effects might
perhaps be identified with what we call voluntary movements; but they do
not concern us at present.
These definitions would have all the precision that could be desired if
the distinction between physical and psychological causation were clear
and sharp. As a matter of fact, however, this distinction is, as yet, by
no means sharp. It is possible that, with fuller knowledge, it will be
found to be no more ultimate than the distinction between the laws of
gases and the laws of rigid bodies. It also suffers from the fact that
an event may be an effect of several causes according to several causal
laws we cannot, in general, point to anything unique as THE cause of
such-and-such an event. And finally it is by no means certain that
the peculiar causal laws which govern mental events are not really
physiological. The law of habit, which is one of the most distinctive,
may be fully explicable in terms of the peculiarities of nervous tissue,
and these peculiarities, in turn, may be explicable by the laws of
physics. It seems, therefore, that we are driven to a different kind of
definition. It is for this reason that it was necessary to develop
the definition of perception. With this definition, we can define a
sensation as the non-mnemic elements in a perception.
When, following our definition, we try to decide what elements in our
experience are of the nature of sensations, we find more difficulty
than might have been expected. Prima facie, everything is sensation that
comes to us through the senses: the sights we see, the sounds we hear,
the smells we smell, and so on; also such things as headache or the
feeling of muscular strain. But in actual fact so much interpretation,
so much of habitual correlation, is mixed with all such experiences,
that the core of pure sensation is only to be extracted by careful
investigation. To take a simple illustration: if you go to the theatre
in your own country, you seem to hear equally well in the stalls or the
dress circle; in either case you think you miss nothing. But if you go
in a foreign country where you have a fair knowledge of the language,
you will seem to have grown partially deaf, and you will find it
necessary to be much nearer the stage than you would need to be in your
own country. The reason is that, in hearing our own language spoken, we
quickly and unconsciously fill out what we really hear with inferences
to what the man must be saying, and we never realize that we have not
heard the words we have merely inferred. In a foreign language, these
inferences are more difficult, and we are more dependent upon actual
sensation. If we found ourselves in a foreign world, where tables looked
like cushions and cushions like tables, we should similarly discover how
much of what we think we see is really inference. Every fairly familiar
sensation is to us a sign of the things that usually go with it, and
many of these things will seem to form part of the sensation. I remember
in the early days of motor-cars being with a friend when a tyre burst
with a loud report. He thought it was a pistol, and supported his
opinion by maintaining that he had seen the flash. But of course there
had been no flash. Nowadays no one sees a flash when a tyre bursts.
In order, therefore, to arrive at what really is sensation in an
occurrence which, at first sight, seems to contain nothing else, we have
to pare away all that is due to habit or expectation or interpretation.
This is a matter for the psychologist, and by no means an easy matter.
For our purposes, it is not important to determine what exactly is the
sensational core in any case; it is only important to notice that
there certainly is a sensational core, since habit, expectation and
interpretation are diversely aroused on diverse occasions, and the
diversity is clearly due to differences in what is presented to
the senses. When you open your newspaper in the morning, the actual
sensations of seeing the print form a very minute part of what goes
on in you, but they are the starting-point of all the rest, and it
is through them that the newspaper is a means of information or
mis-information. Thus, although it may be difficult to determine what
exactly is sensation in any given experience, it is clear that there is
sensation, unless, like Leibniz, we deny all action of the outer world
upon us.
Sensations are obviously the source of our knowledge of the world,
including our own body. It might seem natural to regard a sensation as
itself a cognition, and until lately I did so regard it. When, say, I
see a person I know coming towards me in the street, it SEEMS as
though the mere seeing were knowledge. It is of course undeniable that
knowledge comes THROUGH the seeing, but I think it is a mistake to
regard the mere seeing itself as knowledge. If we are so to regard it,
we must distinguish the seeing from what is seen: we must say that, when
we see a patch of colour of a certain shape, the patch of colour is one
thing and our seeing of it is another. This view, however, demands the
admission of the subject, or act, in the sense discussed in our first
lecture. If there is a subject, it can have a relation to the patch of
colour, namely, the sort of relation which we might call awareness. In
that case the sensation, as a mental event, will consist of awareness of
the colour, while the colour itself will remain wholly physical, and
may be called the sense-datum, to distinguish it from the sensation.
The subject, however, appears to be a logical fiction, like mathematical
points and instants. It is introduced, not because observation reveals
it, but because it is linguistically convenient and apparently demanded
by grammar. Nominal entities of this sort may or may not exist, but
there is no good ground for assuming that they do. The functions that
they appear to perform can always be performed by classes or series or
other logical constructions, consisting of less dubious entities. If we
are to avoid a perfectly gratuitous assumption, we must dispense with
the subject as one of the actual ingredients of the world. But when
we do this, the possibility of distinguishing the sensation from
the sense-datum vanishes; at least I see no way of preserving the
distinction. Accordingly the sensation that we have when we see a patch
of colour simply is that patch of colour, an actual constituent of the
physical world, and part of what physics is concerned with. A patch of
colour is certainly not knowledge, and therefore we cannot say that pure
sensation is cognitive. Through its psychological effects, it is the
cause of cognitions, partly by being itself a sign of things that
are correlated with it, as e.g. sensations of sight and touch are
correlated, and partly by giving rise to images and memories after the
sensation is faded. But in itself the pure sensation is not cognitive.
In the first lecture we considered the view of Brentano, that "we may
define psychical phenomena by saying that they are phenomena which
intentionally contain an object." We saw reasons to reject this view in
general; we are now concerned to show that it must be rejected in the
particular case of sensations. The kind of argument which formerly made
me accept Brentano's view in this case was exceedingly simple. When I
see a patch of colour, it seemed to me that the colour is not psychical,
but physical, while my seeing is not physical, but psychical. Hence
I concluded that the colour is something other than my seeing of
the colour. This argument, to me historically, was directed against
idealism: the emphatic part of it was the assertion that the colour is
physical, not psychical. I shall not trouble you now with the grounds
for holding as against Berkeley that the patch of colour is physical; I
have set them forth before, and I see no reason to modify them. But it
does not follow that the patch of colour is not also psychical, unless
we assume that the physical and the psychical cannot overlap, which I
no longer consider a valid assumption. If we admit--as I think we
should--that the patch of colour may be both physical and psychical, the
reason for distinguishing the sense-datum from the sensation disappears,
and we may say that the patch of colour and our sensation in seeing it
are identical.
This is the view of William James, Professor Dewey, and the American
realists. Perceptions, says Professor Dewey, are not per se cases of
knowledge, but simply natural events with no more knowledge status
than (say) a shower. "Let them [the realists] try the experiment of
conceiving perceptions as pure natural events, not cases of awareness or
apprehension, and they will be surprised to see how little they miss."*
I think he is right in this, except in supposing that the realists will
be surprised. Many of them already hold the view he is advocating, and
others are very sympathetic to it. At any rate, it is the view which I
shall adopt in these lectures.
* Dewey, "Essays in Experimental Logic," pp. 253, 262.
The stuff of the world, so far as we have experience of it, consists, on
the view that I am advocating, of innumerable transient particulars such
as occur in seeing, hearing, etc., together with images more or less
resembling these, of which I shall speak shortly. If physics is true,
there are, besides the particulars that we experience, others, probably
equally (or almost equally) transient, which make up that part of the
material world that does not come into the sort of contact with a
living body that is required to turn it into a sensation. But this topic
belongs to the philosophy of physics, and need not concern us in our
present inquiry.
Sensations are what is common to the mental and physical worlds; they
may be defined as the intersection of mind and matter. This is by no
means a new view; it is advocated, not only by the American authors I
have mentioned, but by Mach in his Analysis of Sensations, which was
published in 1886. The essence of sensation, according to the view I am
advocating, is its independence of past experience. It is a core in our
actual experiences, never existing in isolation except possibly in very
young infants. It is not itself knowledge, but it supplies the data for
our knowledge of the physical world, including our own bodies.
There are some who believe that our mental life is built up out of
sensations alone. This may be true; but in any case I think the only
ingredients required in addition to sensations are images. What images
are, and how they are to be defined, we have now to inquire.
The distinction between images and sensations might seem at first sight
by no means difficult. When we shut our eyes and call up pictures of
familiar scenes, we usually have no difficulty, so long as we remain
awake, in discriminating between what we are imagining and what is
really seen. If we imagine some piece of music that we know, we can go
through it in our mind from beginning to end without any discoverable
tendency to suppose that we are really hearing it. But although such
cases are so clear that no confusion seems possible, there are many
others that are far more difficult, and the definition of images is by
no means an easy problem.
To begin with: we do not always know whether what we are experiencing is
a sensation or an image. The things we see in dreams when our eyes are
shut must count as images, yet while we are dreaming they seem like
sensations. Hallucinations often begin as persistent images, and only
gradually acquire that influence over belief that makes the patient
regard them as sensations. When we are listening for a faint sound--the
striking of a distant clock, or a horse's hoofs on the road--we think
we hear it many times before we really do, because expectation brings
us the image, and we mistake it for sensation. The distinction between
images and sensations is, therefore, by no means always obvious to
inspection.*
* On the distinction between images and sensation, cf.
Semon, "Die mnemischen Empfindungen," pp. 19-20.
We may consider three different ways in which it has been sought to
distinguish images from sensations, namely:
(1) By the less degree of vividness in images;
(2) By our absence of belief in their "physical reality";
(3) By the fact that their causes and effects are different from those
of sensations.
I believe the third of these to be the only universally applicable
criterion. The other two are applicable in very many cases, but
cannot be used for purposes of definition because they are liable to
exceptions. Nevertheless, they both deserve to be carefully considered.
(1) Hume, who gives the names "impressions" and "ideas" to what may,
for present purposes, be identified with our "sensations" and "images,"
speaks of impressions as "those perceptions which enter with most force
and violence" while he defines ideas as "the faint images of these (i.e.
of impressions) in thinking and reasoning." His immediately following
observations, however, show the inadequacy of his criteria of "force"
and "faintness." He says:
"I believe it will not be very necessary to employ many words in
explaining this distinction. Every one of himself will readily perceive
the difference betwixt feeling and thinking. The common degrees of these
are easily distinguished, though it is not impossible but in particular
instances they may very nearly approach to each other. Thus in sleep, in
a fever, in madness, or in any very violent emotions of soul, our ideas
may approach to our impressions; as, on the other hand, it sometimes
happens, that our impressions are so faint and low that we cannot
distinguish them from our ideas. But notwithstanding this near
resemblance in a few instances, they are in general so very different,
that no one can make a scruple to rank them under distinct heads, and
assign to each a peculiar name to mark the difference" ("Treatise of
Human Nature," Part I, Section I).
I think Hume is right in holding that they should be ranked under
distinct heads, with a peculiar name for each. But by his own confession
in the above passage, his criterion for distinguishing them is not
always adequate. A definition is not sound if it only applies in cases
where the difference is glaring: the essential purpose of a definition
is to provide a mark which is applicable even in marginal cases--except,
of course, when we are dealing with a conception, like, e.g. baldness,
which is one of degree and has no sharp boundaries. But so far we have
seen no reason to think that the difference between sensations and
images is only one of degree.
Professor Stout, in his "Manual of Psychology," after discussing various
ways of distinguishing sensations and images, arrives at a view which is
a modification of Hume's. He says (I quote from the second edition):
"Our conclusion is that at bottom the distinction between image
and percept, as respectively faint and vivid states, is based on a
difference of quality. The percept has an aggressiveness which does not
belong to the image. It strikes the mind with varying degrees of force
or liveliness according to the varying intensity of the stimulus. This
degree of force or liveliness is part of what we ordinarily mean by
the intensity of a sensation. But this constituent of the intensity of
sensations is absent in mental imagery"(p. 419).
This view allows for the fact that sensations may reach any degree of
faintness--e.g. in the case of a just visible star or a just audible
sound--without becoming images, and that therefore mere faintness cannot
be the characteristic mark of images. After explaining the sudden shock
of a flash of lightning or a steam-whistle, Stout says that "no mere
image ever does strike the mind in this manner"(p. 417). But I believe
that this criterion fails in very much the same instances as those in
which Hume's criterion fails in its original form. Macbeth speaks of--
that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs
Against the use of nature.
The whistle of a steam-engine could hardly have a stronger effect than
this. A very intense emotion will often bring with it--especially
where some future action or some undecided issue is involved--powerful
compelling images which may determine the whole course of life, sweeping
aside all contrary solicitations to the will by their capacity for
exclusively possessing the mind. And in all cases where images,
originally recognized as such, gradually pass into hallucinations, there
must be just that "force or liveliness" which is supposed to be always
absent from images. The cases of dreams and fever-delirium are as
hard to adjust to Professor Stout's modified criterion as to Hume's. I
conclude therefore that the test of liveliness, however applicable in
ordinary instances, cannot be used to define the differences between
sensations and images.
(2) We might attempt to distinguish images from sensations by our
absence of belief in the "physical reality" of images. When we are aware
that what we are experiencing is an image, we do not give it the kind of
belief that we should give to a sensation: we do not think that it has
the same power of producing knowledge of the "external world." Images
are "imaginary"; in SOME sense they are "unreal." But this difference
is hard to analyse or state correctly. What we call the "unreality" of
images requires interpretation it cannot mean what would be expressed
by saying "there's no such thing." Images are just as truly part of the
actual world as sensations are. All that we really mean by calling an
image "unreal" is that it does not have the concomitants which it would
have if it were a sensation. When we call up a visual image of a chair,
we do not attempt to sit in it, because we know that, like Macbeth's
dagger, it is not "sensible to feeling as to sight"--i.e. it does not
have the correlations with tactile sensations which it would have if it
were a visual sensation and not merely a visual image. But this means
that the so-called "unreality" of images consists merely in their not
obeying the laws of physics, and thus brings us back to the causal
distinction between images and sensations.
This view is confirmed by the fact that we only feel images to be
"unreal" when we already know them to be images. Images cannot be
defined by the FEELING of unreality, because when we falsely believe an
image to be a sensation, as in the case of dreams, it FEELS just as real
as if it were a sensation. Our feeling of unreality results from our
having already realized that we are dealing with an image, and cannot
therefore be the definition of what we mean by an image. As soon as an
image begins to deceive us as to its status, it also deceives us as to
its correlations, which are what we mean by its "reality."
(3) This brings us to the third mode of distinguishing images from
sensations, namely, by their causes and effects. I believe this to be
the only valid ground of distinction. James, in the passage about the
mental fire which won't burn real sticks, distinguishes images by their
effects, but I think the more reliable distinction is by their causes.
Professor Stout (loc. cit., p. 127) says: "One characteristic mark of
what we agree in calling sensation is its mode of production. It is
caused by what we call a STIMULUS. A stimulus is always some condition
external to the nervous system itself and operating upon it." I think
that this is the correct view, and that the distinction between images
and sensations can only be made by taking account of their causation.
Sensations come through sense-organs, while images do not. We cannot
have visual sensations in the dark, or with our eyes shut, but we can
very well have visual images under these circumstances. Accordingly
images have been defined as "centrally excited sensations," i.e.
sensations which have their physiological cause in the brain only, not
also in the sense-organs and the nerves that run from the sense-organs
to the brain. I think the phrase "centrally excited sensations" assumes
more than is necessary, since it takes it for granted that an image must
have a proximate physiological cause. This is probably true, but it is
an hypothesis, and for our purposes an unnecessary one. It would seem to
fit better with what we can immediately observe if we were to say that
an image is occasioned, through association, by a sensation or another
image, in other words that it has a mnemic cause--which does not prevent
it from also having a physical cause. And I think it will be found that
the causation of an image always proceeds according to mnemic laws, i.e.
that it is governed by habit and past experience. If you listen to a man
playing the pianola without looking at him, you will have images of his
hands on the keys as if he were playing the piano; if you suddenly look
at him while you are absorbed in the music, you will experience a shock
of surprise when you notice that his hands are not touching the notes.
Your image of his hands is due to the many times that you have heard
similar sounds and at the same time seen the player's hands on the
piano. When habit and past experience play this part, we are in the
region of mnemic as opposed to ordinary physical causation. And I think
that, if we could regard as ultimately valid the difference between
physical and mnemic causation, we could distinguish images from
sensations as having mnemic causes, though they may also have physical
causes. Sensations, on the other hand, will only have physical causes.
However this may be, the practically effective distinction between
sensations and images is that in the causation of sensations, but not
of images, the stimulation of nerves carrying an effect into the brain,
usually from the surface of the body, plays an essential part. And
this accounts for the fact that images and sensations cannot always be
distinguished by their intrinsic nature.
Images also differ from sensations as regards their effects. Sensations,
as a rule, have both physical and mental effects. As you watch the train
you meant to catch leaving the station, there are both the successive
positions of the train (physical effects) and the successive waves
of fury and disappointment (mental effects). Images, on the contrary,
though they MAY produce bodily movements, do so according to mnemic
laws, not according to the laws of physics. All their effects, of
whatever nature, follow mnemic laws. But this difference is less
suitable for definition than the difference as to causes.
Professor Watson, as a logical carrying-out of his behaviourist theory,
denies altogether that there are any observable phenomena such as
images are supposed to be. He replaces them all by faint sensations, and
especially by pronunciation of words sotto voce. When we "think" of a
table (say), as opposed to seeing it, what happens, according to him, is
usually that we are making small movements of the throat and tongue
such as would lead to our uttering the word "table" if they were more
pronounced. I shall consider his view again in connection with words;
for the present I am only concerned to combat his denial of images. This
denial is set forth both in his book on "Behavior" and in an article
called "Image and Affection in Behavior" in the "Journal of Philosophy,
Psychology and Scientific Methods," vol. x (July, 1913). It seems to me
that in this matter he has been betrayed into denying plain facts in
the interests of a theory, namely, the supposed impossibility of
introspection. I dealt with the theory in Lecture VI; for the present I
wish to reinforce the view that the facts are undeniable.
Images are of various sorts, according to the nature of the sensations
which they copy. Images of bodily movements, such as we have when we
imagine moving an arm or, on a smaller scale, pronouncing a word,
might possibly be explained away on Professor Watson's lines, as really
consisting in small incipient movements such as, if magnified and
prolonged, would be the movements we are said to be imagining. Whether
this is the case or not might even be decided experimentally. If there
were a delicate instrument for recording small movements in the mouth
and throat, we might place such an instrument in a person's mouth and
then tell him to recite a poem to himself, as far as possible only in
imagination. I should not be at all surprised if it were found that
actual small movements take place while he is "mentally" saying over
the verses. The point is important, because what is called "thought"
consists mainly (though I think not wholly) of inner speech. If
Professor Watson is right as regards inner speech, this whole region
is transferred from imagination to sensation. But since the question
is capable of experimental decision, it would be gratuitous rashness to
offer an opinion while that decision is lacking.
But visual and auditory images are much more difficult to deal with in
this way, because they lack the connection with physical events in the
outer world which belongs to visual and auditory sensations. Suppose,
for example, that I am sitting in my room, in which there is an empty
arm-chair. I shut my eyes, and call up a visual image of a friend
sitting in the arm-chair. If I thrust my image into the world of
physics, it contradicts all the usual physical laws. My friend reached
the chair without coming in at the door in the usual way; subsequent
inquiry will show that he was somewhere else at the moment. If regarded
as a sensation, my image has all the marks of the supernatural. My
image, therefore, is regarded as an event in me, not as having that
position in the orderly happenings of the public world that belongs to
sensations. By saying that it is an event in me, we leave it possible
that it may be PHYSIOLOGICALLY caused: its privacy may be only due to
its connection with my body. But in any case it is not a public event,
like an actual person walking in at the door and sitting down in
my chair. And it cannot, like inner speech, be regarded as a SMALL
sensation, since it occupies just as large an area in my visual field as
the actual sensation would do.
Professor Watson says: "I should throw out imagery altogether
and attempt to show that all natural thought goes on in terms of
sensori-motor processes in the larynx." This view seems to me flatly to
contradict experience. If you try to persuade any uneducated person that
she cannot call up a visual picture of a friend sitting in a chair, but
can only use words describing what such an occurrence would be like,
she will conclude that you are mad. (This statement is based upon
experiment.) Galton, as every one knows, investigated visual imagery,
and found that education tends to kill it: the Fellows of the Royal
Society turned out to have much less of it than their wives. I see no
reason to doubt his conclusion that the habit of abstract pursuits makes
learned men much inferior to the average in power of visualizing, and
much more exclusively occupied with words in their "thinking." And
Professor Watson is a very learned man.
I shall henceforth assume that the existence of images is admitted, and
that they are to be distinguished from sensations by their causes,
as well as, in a lesser degree, by their effects. In their intrinsic
nature, though they often differ from sensations by being more dim
or vague or faint, yet they do not always or universally differ from
sensations in any way that can be used for defining them. Their privacy
need form no bar to the scientific study of them, any more than the
privacy of bodily sensations does. Bodily sensations are admitted by
even the most severe critics of introspection, although, like images,
they can only be observed by one observer. It must be admitted, however,
that the laws of the appearance and disappearance of images are little
known and difficult to discover, because we are not assisted, as in the
case of sensations, by our knowledge of the physical world.
There remains one very important point concerning images, which will
occupy us much hereafter, and that is, their resemblance to previous
sensations. They are said to be "copies" of sensations, always as
regards the simple qualities that enter into them, though not always
as regards the manner in which these are put together. It is generally
believed that we cannot imagine a shade of colour that we have never
seen, or a sound that we have never heard. On this subject Hume is the
classic. He says, in the definitions already quoted:
"Those perceptions, which enter with most force and violence, we may
name IMPRESSIONS; and under this name I comprehend all our sensations,
passions and emotions, as they make their first appearance in the soul.
By IDEAS I mean the faint images of these in thinking and reasoning."
He next explains the difference between simple and complex ideas, and
explains that a complex idea may occur without any similar complex
impression. But as regards simple ideas, he states that "every simple
idea has a simple impression, which resembles it, and every simple
impression a correspondent idea." He goes on to enunciate the general
principle "that all our simple ideas in their first appearance are
derived from simple impressions, which are correspondent to them, and
which they exactly represent" ("Treatise of Human Nature," Part I,
Section I).
It is this fact, that images resemble antecedent sensations, which
enables us to call them images "of" this or that. For the understanding
of memory, and of knowledge generally, the recognizable resemblance of
images and sensations is of fundamental importance.
There are difficulties in establishing Hume's principles, and doubts
as to whether it is exactly true. Indeed, he himself signalized an
exception immediately after stating his maxim. Nevertheless, it is
impossible to doubt that in the main simple images are copies of similar
simple sensations which have occurred earlier, and that the same is true
of complex images in all cases of memory as opposed to mere imagination.
Our power of acting with reference to what is sensibly absent is largely
due to this characteristic of images, although, as education advances,
images tend to be more and more replaced by words. We shall have much
to say in the next two lectures on the subject of images as copies of
sensations. What has been said now is merely by way of reminder that
this is their most notable characteristic.
I am by no means confident that the distinction between images and
sensations is ultimately valid, and I should be glad to be convinced
that images can be reduced to sensations of a peculiar kind. I think it
is clear, however, that, at any rate in the case of auditory and visual
images, they do differ from ordinary auditory and visual sensations, and
therefore form a recognizable class of occurrences, even if it should
prove that they can be regarded as a sub-class of sensations. This is
all that is necessary to validate the use of images to be made in the
sequel.