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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
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