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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”;
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