derivative but logically primitive beliefs, we shall find that most, if
not all, are to be classed as soft data. They may be found, on
reflection, to be capable of logical proof, and they then again become
believed, but no longer as data. As data, though entitled to a certain
limited respect, they cannot be placed on a level with the facts of
sense or the laws of logic. The kind of respect which they deserve seems
to me such as to warrant us in hoping, though not too confidently, that
the hard data may prove them to be at least probable. Also, if the hard
data are found to throw no light whatever upon their truth or falsehood,
we are justified, I think, in giving rather more weight to the
hypothesis of their truth than to the hypothesis of their falsehood. For
the present, however, let us confine ourselves to the hard data, with a
view to discovering what sort of world can be constructed by their means
alone.
Our data now are primarily the facts of sense (_i.e._ of _our own_
sense-data) and the laws of logic. But even the severest scrutiny will
allow some additions to this slender stock. Some facts of
memory--especially of recent memory--seem to have the highest degree of
certainty. Some introspective facts are as certain as any facts of
sense. And facts of sense themselves must, for our present purposes, be
interpreted with a certain latitude. Spatial and temporal relations must
sometimes be included, for example in the case of a swift motion falling
wholly within the specious present. And some facts of comparison, such
as the likeness or unlikeness of two shades of colour, are certainly to
be included among hard data. Also we must remember that the distinction
of hard and soft data is psychological and subjective, so that, if there
are other minds than our own--which at our present stage must be held
doubtful--the catalogue of hard data may be different for them from what
it is for us.
Certain common beliefs are undoubtedly excluded from hard data. Such is
the belief which led us to introduce the distinction, namely, that
sensible objects in general persist when we are not perceiving them.
Such also is the belief in other people's minds: this belief is
obviously derivative from our perception of their bodies, and is felt to
demand logical justification as soon as we become aware of its
derivativeness. Belief in what is reported by the testimony of others,
including all that we learn from books, is of course involved in the
doubt as to whether other people have minds at all. Thus the world from
which our reconstruction is to begin is very fragmentary. The best we
can say for it is that it is slightly more extensive than the world at
which Descartes arrived by a similar process, since that world contained
nothing except himself and his thoughts.
We are now in a position to understand and state the problem of our
knowledge of the external world, and to remove various misunderstandings
which have obscured the meaning of the problem. The problem really is:
Can the existence of anything other than our own hard data be inferred
from the existence of those data? But before considering this problem,
let us briefly consider what the problem is _not_.
When we speak of the "external" world in this discussion, we must not
mean "spatially external," unless "space" is interpreted in a peculiar
and recondite manner. The immediate objects of sight, the coloured
surfaces which make up the visible world, are spatially external in the
natural meaning of this phrase. We feel them to be "there" as opposed to
"here"; without making any assumption of an existence other than hard
data, we can more or less estimate the distance of a coloured surface.
It seems probable that distances, provided they are not too great, are
actually given more or less roughly in sight; but whether this is the
case or not, ordinary distances can certainly be estimated approximately
by means of the data of sense alone. The immediately given world is
spatial, and is further not wholly contained within our own bodies. Thus
our knowledge of what is external in this sense is not open to doubt.
Another form in which the question is often put is: "Can we know of the
existence of any reality which is independent of ourselves?" This form
of the question suffers from the ambiguity of the two words
"independent" and "self." To take the Self first: the question as to
what is to be reckoned part of the Self and what is not, is a very
difficult one. Among many other things which we may mean by the Self,
two may be selected as specially important, namely, (1) the bare subject
which thinks and is aware of objects, (2) the whole assemblage of things
that would necessarily cease to exist if our lives came to an end. The
bare subject, if it exists at all, is an inference, and is not part of
the data; therefore this meaning of Self may be ignored in our present
inquiry. The second meaning is difficult to make precise, since we
hardly know what things depend upon our lives for their existence. And
in this form, the definition of Self introduces the word "depend," which
raises the same questions as are raised by the word "independent." Let
us therefore take up the word "independent," and return to the Self
later.
When we say that one thing is "independent" of another, we may mean
either that it is logically possible for the one to exist without the
other, or that there is no causal relation between the two such that the
one only occurs as the effect of the other. The only way, so far as I
know, in which one thing can be _logically_ dependent upon another is
when the other is _part_ of the one. The existence of a book, for
example, is logically dependent upon that of its pages: without the
pages there would be no book. Thus in this sense the question, "Can we
know of the existence of any reality which is independent of ourselves?"
reduces to the question, "Can we know of the existence of any reality of
which our Self is not part?" In this form, the question brings us back
to the problem of defining the Self; but I think, however the Self may
be defined, even when it is taken as the bare subject, it cannot be
supposed to be part of the immediate object of sense; thus in this form
of the question we must admit that we can know of the existence of
realities independent of ourselves.
The question of causal dependence is much more difficult. To know that
one kind of thing is causally independent of another, we must know that
it actually occurs without the other. Now it is fairly obvious that,
whatever legitimate meaning we give to the Self, our thoughts and
feelings are causally dependent upon ourselves, _i.e._ do not occur when
there is no Self for them to belong to. But in the case of objects of
sense this is not obvious; indeed, as we saw, the common-sense view is
that such objects persist in the absence of any percipient. If this is
the case, then they are causally independent of ourselves; if not, not.
Thus in this form the question reduces to the question whether we can
know that objects of sense, or any other objects not our own thoughts
and feelings, exist at times when we are not perceiving them. This form,
in which the difficult word "independent" no longer occurs, is the form
in which we stated the problem a minute ago.
Our question in the above form raises two distinct problems, which it is
important to keep separate. First, can we know that objects of sense, or
very similar objects, exist at times when we are not perceiving them?
Secondly, if this cannot be known, can we know that other objects,
inferable from objects of sense but not necessarily resembling them,
exist either when we are perceiving the objects of sense or at any other
time? This latter problem arises in philosophy as the problem of the
"thing in itself," and in science as the problem of matter as assumed in
physics. We will consider this latter problem first.
Owing to the fact that we feel passive in sensation, we naturally
suppose that our sensations have outside causes. Now it is necessary
here first of all to distinguish between (1) our sensation, which is a
mental event consisting in our being aware of a sensible object, and (2)
the sensible object of which we are aware in sensation. When I speak of
the sensible object, it must be understood that I do not mean such a
thing as a table, which is both visible and tangible, can be seen by
many people at once, and is more or less permanent. What I mean is just
that patch of colour which is momentarily seen when we look at the
table, or just that particular hardness which is felt when we press it,
or just that particular sound which is heard when we rap it. Each of
these I call a sensible object, and our awareness of it I call a
sensation. Now our sense of passivity, if it really afforded any
argument, would only tend to show that the _sensation_ has an outside
cause; this cause we should naturally seek in the sensible object. Thus
there is no good reason, so far, for supposing that sensible objects
must have outside causes. But both the thing-in-itself of philosophy and
the matter of physics present themselves as outside causes of the
sensible object as much as of the sensation. What are the grounds for
this common opinion?
In each case, I think, the opinion has resulted from the combination of
a belief that _something_ which can persist independently of our
consciousness makes itself known in sensation, with the fact that our
sensations often change in ways which seem to depend upon us rather than
upon anything which would be supposed to persist independently of us. At
first, we believe unreflectingly that everything is as it seems to be,
and that, if we shut our eyes, the objects we had been seeing remain as
they were though we no longer see them. But there are arguments against
this view, which have generally been thought conclusive. It is
extraordinarily difficult to see just what the arguments prove; but if
we are to make any progress with the problem of the external world, we
must try to make up our minds as to these arguments.
A table viewed from one place presents a different appearance from that
which it presents from another place. This is the language of common
sense, but this language already assumes that there is a real table of
which we see the appearances. Let us try to state what is known in terms
of sensible objects alone, without any element of hypothesis. We find
that as we walk round the table, we perceive a series of gradually
changing visible objects. But in speaking of "walking round the table,"
we have still retained the hypothesis that there is a single table
connected with all the appearances. What we ought to say is that, while
we have those muscular and other sensations which make us say we are
walking, our visual sensations change in a continuous way, so that, for
example, a striking patch of colour is not suddenly replaced by
something wholly different, but is replaced by an insensible gradation
of slightly different colours with slightly different shapes. This is
what we really know by experience, when we have freed our minds from the
assumption of permanent "things" with changing appearances. What is
really known is a correlation of muscular and other bodily sensations
with changes in visual sensations.
But walking round the table is not the only way of altering its
appearance. We can shut one eye, or put on blue spectacles, or look
through a microscope. All these operations, in various ways, alter the
visual appearance which we call that of the table. More distant objects
will also alter their appearance if (as we say) the state of the
atmosphere changes--if there is fog or rain or sunshine. Physiological
changes also alter the appearances of things. If we assume the world of
common sense, all these changes, including those attributed to
physiological causes, are changes in the intervening medium. It is not
quite so easy as in the former case to reduce this set of facts to a
form in which nothing is assumed beyond sensible objects. Anything
intervening between ourselves and what we see must be invisible: our
view in every direction is bounded by the nearest visible object. It
might be objected that a dirty pane of glass, for example, is visible
although we can see things through it. But in this case we really see a
spotted patchwork: the dirtier specks in the glass are visible, while
the cleaner parts are invisible and allow us to see what is beyond. Thus
the discovery that the intervening medium affects the appearances of
things cannot be made by means of the sense of sight alone.
Let us take the case of the blue spectacles, which is the simplest, but
may serve as a type for the others. The frame of the spectacles is of
course visible, but the blue glass, if it is clean, is not visible. The
blueness, which we say is in the glass, appears as being in the objects
seen through the glass. The glass itself is known by means of the sense
of touch. In order to know that it is between us and the objects seen
through it, we must know how to correlate the space of touch with the
space of sight. This correlation itself, when stated in terms of the
data of sense alone, is by no means a simple matter. But it presents no
difficulties of principle, and may therefore be supposed accomplished.
When it has been accomplished, it becomes possible to attach a meaning
to the statement that the blue glass, which we can touch, is between us
and the object seen, as we say, "through" it.
But we have still not reduced our statement completely to what is
actually given in sense. We have fallen into the assumption that the
object of which we are conscious when we touch the blue spectacles still
exists after we have ceased to touch them. So long as we are touching
them, nothing except our finger can be seen through the part touched,
which is the only part where we immediately know that there is
something. If we are to account for the blue appearance of objects other
than the spectacles, when seen through them, it might seem as if we must
assume that the spectacles still exist when we are not touching them;
and if this assumption really is necessary, our main problem is
answered: we have means of knowing of the present existence of objects
not given in sense, though of the same kind as objects formerly given in
sense.
It may be questioned, however, whether this assumption is actually
unavoidable, though it is unquestionably the most natural one to make.
We may say that the object of which we become aware when we touch the
spectacles continues to have effects afterwards, though perhaps it no
longer exists. In this view, the supposed continued existence of
sensible objects after they have ceased to be sensible will be a
fallacious inference from the fact that they still have effects. It is
often supposed that nothing which has ceased to exist can continue to
have effects, but this is a mere prejudice, due to a wrong conception of
causality. We cannot, therefore, dismiss our present hypothesis on the
ground of _a priori_ impossibility, but must examine further whether it
can really account for the facts.
It may be said that our hypothesis is useless in the case when the blue
glass is never touched at all. How, in that case, are we to account for
the blue appearance of objects? And more generally, what are we to make
of the hypothetical sensations of touch which we associate with
untouched visible objects, which we know would be verified if we chose,
though in fact we do not verify them? Must not these be attributed to
permanent possession, by the objects, of the properties which touch
would reveal?
Let us consider the more general question first. Experience has taught
us that where we see certain kinds of coloured surfaces we can, by
touch, obtain certain expected sensations of hardness or softness,
tactile shape, and so on. This leads us to believe that what is seen is
usually tangible, and that it has, whether we touch it or not, the
hardness or softness which we should expect to feel if we touched it.
But the mere fact that we are able to infer what our tactile sensations
would be shows that it is not logically necessary to assume tactile
qualities before they are felt. All that is really known is that the
visual appearance in question, together with touch, will lead to certain
sensations, which can necessarily be determined in terms of the visual
appearance, since otherwise they could not be inferred from it.
We can now give a statement of the experienced facts concerning the blue
spectacles, which will supply an interpretation of common-sense beliefs
without assuming anything beyond the existence of sensible objects at
the times when they are sensible. By experience of the correlation of
touch and sight sensations, we become able to associate a certain place
in touch-space with a certain corresponding place in sight-space.
Sometimes, namely in the case of transparent things, we find that there
is a tangible object in a touch-place without there being any visible
object in the corresponding sight-place. But in such a case as that of
the blue spectacles, we find that whatever object is visible beyond the
empty sight-place in the same line of sight has a different colour from
what it has when there is no tangible object in the intervening
touch-place; and as we move the tangible object in touch-space, the blue
patch moves in sight-space. If now we find a blue patch moving in this
way in sight-space, when we have no sensible experience of an
intervening tangible object, we nevertheless infer that, if we put our
hand at a certain place in touch-space, we should experience a certain
touch-sensation. If we are to avoid non-sensible objects, this must be
taken as the whole of our meaning when we say that the blue spectacles
are in a certain place, though we have not touched them, and have only
seen other things rendered blue by their interposition.
I think it may be laid down quite generally that, _in so far_ as physics
or common sense is verifiable, it must be capable of interpretation in
terms of actual sense-data alone. The reason for this is simple.
Verification consists always in the occurrence of an expected
sense-datum. Astronomers tell us there will be an eclipse of the moon:
we look at the moon, and find the earth's shadow biting into it, that is
to say, we see an appearance quite different from that of the usual full
moon. Now if an expected sense-datum constitutes a verification, what
was asserted must have been about sense-data; or, at any rate, if part
of what was asserted was not about sense-data, then only the other part
has been verified. There is in fact a certain regularity or conformity
to law about the occurrence of sense-data, but the sense-data that occur
at one time are often causally connected with those that occur at quite
other times, and not, or at least not very closely, with those that
occur at neighbouring times. If I look at the moon and immediately
afterwards hear a train coming, there is no very close causal connection
between my two sense-data; but if I look at the moon on two nights a
week apart, there is a very close causal connection between the two
sense-data. The simplest, or at least the easiest, statement of the
connection is obtained by imagining a "real" moon which goes on whether
I look at it or not, providing a series of _possible_ sense-data of
which only those are actual which belong to moments when I choose to
look at the moon.
But the degree of verification obtainable in this way is very small. It
must be remembered that, at our present level of doubt, we are not at
liberty to accept testimony. When we hear certain noises, which are
those we should utter if we wished to express a certain thought, we
assume that that thought, or one very like it, has been in another mind,
and has given rise to the expression which we hear. If at the same time
we see a body resembling our own, moving its lips as we move ours when
we speak, we cannot resist the belief that it is alive, and that the
feelings inside it continue when we are not looking at it. When we see
our friend drop a weight upon his toe, and hear him say--what we should
say in similar circumstances, the phenomena _can_ no doubt be explained
without assuming that he is anything but a series of shapes and noises
seen and heard by us, but practically no man is so infected with
philosophy as not to be quite certain that his friend has felt the same
kind of pain as he himself would feel. We will consider the legitimacy
of this belief presently; for the moment, I only wish to point out that
it needs the same kind of justification as our belief that the moon
exists when we do not see it, and that, without it, testimony heard or
read is reduced to noises and shapes, and cannot be regarded as evidence
of the facts which it reports. The verification of physics which is
possible at our present level is, therefore, only that degree of
verification which is possible by one man's unaided observations, which
will not carry us very far towards the establishment of a whole science.
Before proceeding further, let us summarise the argument so far as it
has gone. The problem is: "Can the existence of anything other than our
own hard data be inferred from these data?" It is a mistake to state the
problem in the form: "Can we know of the existence of anything other
than ourselves and our states?" or: "Can we know of the existence of
anything independent of ourselves?" because of the extreme difficulty of
defining "self" and "independent" precisely. The felt passivity of
sensation is irrelevant, since, even if it proved anything, it could
only prove that sensations are caused by sensible objects. The natural
_naive_ belief is that things seen persist, when unseen, exactly or
approximately as they appeared when seen; but this belief tends to be
dispelled by the fact that what common sense regards as the appearance
of one object changes with what common sense regards as changes in the
point of view and in the intervening medium, including in the latter our
own sense-organs and nerves and brain. This fact, as just stated,
assumes, however, the common-sense world of stable objects which it
professes to call in question; hence, before we can discover its precise
bearing on our problem, we must find a way of stating it which does not
involve any of the assumptions which it is designed to render doubtful.
What we then find, as the bare outcome of experience, is that gradual
changes in certain sense-data are correlated with gradual changes in
certain others, or (in the case of bodily motions) with the other
sense-data themselves.
The assumption that sensible objects persist after they have ceased to
be sensible--for example, that the hardness of a visible body, which has
been discovered by touch, continues when the body is no longer
touched--may be replaced by the statement that the _effects_ of sensible
objects persist, _i.e._ that what happens now can only be accounted for,
in many cases, by taking account of what happened at an earlier time.
Everything that one man, by his own personal experience, can verify in
the account of the world given by common sense and physics, will be
explicable by some such means, since verification consists merely in the
occurrence of an expected sense-datum. But what depends upon testimony,
whether heard or read, cannot be explained in this way, since testimony
depends upon the existence of minds other than our own, and thus
requires a knowledge of something not given in sense. But before
examining the question of our knowledge of other minds, let us return to
the question of the thing-in-itself, namely, to the theory that what
exists at times when we are not perceiving a given sensible object is
something quite unlike that object, something which, together with us
and our sense-organs, causes our sensations, but is never itself given
in sensation.