fairly natural outcome of the difficulties due to the changing
appearances of what is supposed to be one object. It is supposed that
the table (for example) causes our sense-data of sight and touch, but
must, since these are altered by the point of view and the intervening
medium, be quite different from the sense-data to which it gives rise.
There is, in this theory, a tendency to a confusion from which it
derives some of its plausibility, namely, the confusion between a
sensation as a psychical occurrence and its object. A patch of colour,
even if it only exists when it is seen, is still something quite
different from the seeing of it: the seeing of it is mental, but the
patch of colour is not. This confusion, however, can be avoided without
our necessarily abandoning the theory we are examining. The objection to
it, I think, lies in its failure to realise the radical nature of the
reconstruction demanded by the difficulties to which it points. We
cannot speak legitimately of changes in the point of view and the
intervening medium until we have already constructed some world more
stable than that of momentary sensation. Our discussion of the blue
spectacles and the walk round the table has, I hope, made this clear.
But what remains far from clear is the nature of the reconstruction
required.
Although we cannot rest content with the above theory, in the terms in
which it is stated, we must nevertheless treat it with a certain
respect, for it is in outline the theory upon which physical science and
physiology are built, and it must, therefore, be susceptible of a true
interpretation. Let us see how this is to be done.
The first thing to realise is that there are no such things as
"illusions of sense." Objects of sense, even when they occur in dreams,
are the most indubitably real objects known to us. What, then, makes us
call them unreal in dreams? Merely the unusual nature of their
connection with other objects of sense. I dream that I am in America,
but I wake up and find myself in England without those intervening days
on the Atlantic which, alas! are inseparably connected with a "real"
visit to America. Objects of sense are called "real" when they have the
kind of connection with other objects of sense which experience has led
us to regard as normal; when they fail in this, they are called
"illusions." But what is illusory is only the inferences to which they
give rise; in themselves, they are every bit as real as the objects of
waking life. And conversely, the sensible objects of waking life must
not be expected to have any more intrinsic reality than those of dreams.
Dreams and waking life, in our first efforts at construction, must be
treated with equal respect; it is only by some reality not _merely_
sensible that dreams can be condemned.
Accepting the indubitable momentary reality of objects of sense, the
next thing to notice is the confusion underlying objections derived from
their changeableness. As we walk round the table, its aspect changes;
but it is thought impossible to maintain either that the table changes,
or that its various aspects can all "really" exist in the same place. If
we press one eyeball, we shall see two tables; but it is thought
preposterous to maintain that there are "really" two tables. Such
arguments, however, seem to involve the assumption that there can be
something more real than objects of sense. If we see two tables, then
there _are_ two visual tables. It is perfectly true that, at the same
moment, we may discover by touch that there is only one tactile table.
This makes us declare the two visual tables an illusion, because usually
one visual object corresponds to one tactile object. But all that we are
warranted in saying is that, in this case, the manner of correlation of
touch and sight is unusual. Again, when the aspect of the table changes
as we walk round it, and we are told there cannot be so many different
aspects in the same place, the answer is simple: what does the critic of
the table mean by "the same place"? The use of such a phrase presupposes
that all our difficulties have been solved; as yet, we have no right to
speak of a "place" except with reference to one given set of momentary
sense-data. When all are changed by a bodily movement, no place remains
the same as it was. Thus the difficulty, if it exists, has at least not
been rightly stated.
We will now make a new start, adopting a different method. Instead of
inquiring what is the minimum of assumption by which we can explain the
world of sense, we will, in order to have a model hypothesis as a help
for the imagination, construct one possible (not necessary) explanation
of the facts. It may perhaps then be possible to pare away what is
superfluous in our hypothesis, leaving a residue which may be regarded
as the abstract answer to our problem.
Let us imagine that each mind looks out upon the world, as in Leibniz's
monadology, from a point of view peculiar to itself; and for the sake of
simplicity let us confine ourselves to the sense of sight, ignoring
minds which are devoid of this sense. Each mind sees at each moment an
immensely complex three-dimensional world; but there is absolutely
nothing which is seen by two minds simultaneously. When we say that two
people see the same thing, we always find that, owing to difference of
point of view, there are differences, however slight, between their
immediate sensible objects. (I am here assuming the validity of
testimony, but as we are only constructing a _possible_ theory, that is
a legitimate assumption.) The three-dimensional world seen by one mind
therefore contains no place in common with that seen by another, for
places can only be constituted by the things in or around them. Hence we
may suppose, in spite of the differences between the different worlds,
that each exists entire exactly as it is perceived, and might be exactly
as it is even if it were not perceived. We may further suppose that
there are an infinite number of such worlds which are in fact
unperceived. If two men are sitting in a room, two somewhat similar
worlds are perceived by them; if a third man enters and sits between
them, a third world, intermediate between the two previous worlds,
begins to be perceived. It is true that we cannot reasonably suppose
just this world to have existed before, because it is conditioned by the
sense-organs, nerves, and brain of the newly arrived man; but we can
reasonably suppose that _some_ aspect of the universe existed from that
point of view, though no one was perceiving it. The system consisting of
all views of the universe perceived and unperceived, I shall call the
system of "perspectives"; I shall confine the expression "private
worlds" to such views of the universe as are actually perceived. Thus a
"private world" is a perceived "perspective"; but there may be any
number of unperceived perspectives.
Two men are sometimes found to perceive very similar perspectives, so
similar that they can use the same words to describe them. They say they
see the same table, because the differences between the two tables they
see are slight and not practically important. Thus it is possible,
sometimes, to establish a correlation by similarity between a great many
of the things of one perspective, and a great many of the things of
another. In case the similarity is very great, we say the points of view
of the two perspectives are near together in space; but this space in
which they are near together is totally different from the spaces inside
the two perspectives. It is a relation between the perspectives, and is
not in either of them; no one can perceive it, and if it is to be known
it can be only by inference. Between two perceived perspectives which
are similar, we can imagine a whole series of other perspectives, some
at least unperceived, and such that between any two, however similar,
there are others still more similar. In this way the space which
consists of relations between perspectives can be rendered continuous,
and (if we choose) three-dimensional.
We can now define the momentary common-sense "thing," as opposed to its
momentary appearances. By the similarity of neighbouring perspectives,
many objects in the one can be correlated with objects in the other,
namely, with the similar objects. Given an object in one perspective,
form the system of all the objects correlated with it in all the
perspectives; that system may be identified with the momentary
common-sense "thing." Thus an aspect of a "thing" is a member of the
system of aspects which _is_ the "thing" at that moment. (The
correlation of the times of different perspectives raises certain
complications, of the kind considered in the theory of relativity; but
we may ignore these at present.) All the aspects of a thing are real,
whereas the thing is a mere logical construction. It has, however, the
merit of being neutral as between different points of view, and of being
visible to more than one person, in the only sense in which it can ever
be visible, namely, in the sense that each sees one of its aspects.
It will be observed that, while each perspective contains its own space,
there is only one space in which the perspectives themselves are the
elements. There are as many private spaces as there are perspectives;
there are therefore at least as many as there are percipients, and there
may be any number of others which have a merely material existence and
are not seen by anyone. But there is only one perspective-space, whose
elements are single perspectives, each with its own private space. We
have now to explain how the private space of a single perspective is
correlated with part of the one all-embracing perspective space.
Perspective space is the system of "points of view" of private spaces
(perspectives), or, since "points of view" have not been defined, we may
say it is the system of the private spaces themselves. These private
spaces will each count as one point, or at any rate as one element, in
perspective space. They are ordered by means of their similarities.
Suppose, for example, that we start from one which contains the
appearance of a circular disc, such as would be called a penny, and
suppose this appearance, in the perspective in question, is circular,
not elliptic. We can then form a whole series of perspectives containing
a graduated series of circular aspects of varying sizes: for this
purpose we only have to move (as we say) towards the penny or away from
it. The perspectives in which the penny looks circular will be said to
lie on a straight line in perspective space, and their order on this
line will be that of the sizes of the circular aspects. Moreover--though
this statement must be noticed and subsequently examined--the
perspectives in which the penny looks big will be said to be nearer to
the penny than those in which it looks small. It is to be remarked also
that any other "thing" than our penny might have been chosen to define
the relations of our perspectives in perspective space, and that
experience shows that the same spatial order of perspectives would have
resulted.
In order to explain the correlation of private spaces with perspective
space, we have first to explain what is meant by "the place (in
perspective space) where a thing is." For this purpose, let us again
consider the penny which appears in many perspectives. We formed a
straight line of perspectives in which the penny looked circular, and we
agreed that those in which it looked larger were to be considered as
nearer to the penny. We can form another straight line of perspectives
in which the penny is seen end-on and looks like a straight line of a
certain thickness. These two lines will meet in a certain place in
perspective space, _i.e._ in a certain perspective, which may be defined
as "the place (in perspective space) where the penny is." It is true
that, in order to prolong our lines until they reach this place, we
shall have to make use of other things besides the penny, because, so
far as experience goes, the penny ceases to present any appearance after
we have come so near to it that it touches the eye. But this raises no
real difficulty, because the spatial order of perspectives is found
empirically to be independent of the particular "things" chosen for
defining the order. We can, for example, remove our penny and prolong
each of our two straight lines up to their intersection by placing other
pennies further off in such a way that the aspects of the one are
circular where those of our original penny were circular, and the
aspects of the other are straight where those of our original penny were
straight. There will then be just one perspective in which one of the
new pennies looks circular and the other straight. This will be, by
definition, the place where the original penny was in perspective space.
The above is, of course, only a first rough sketch of the way in which
our definition is to be reached. It neglects the size of the penny, and
it assumes that we can remove the penny without being disturbed by any
simultaneous changes in the positions of other things. But it is plain
that such niceties cannot affect the principle, and can only introduce
complications in its application.
Having now defined the perspective which is the place where a given
thing is, we can understand what is meant by saying that the
perspectives in which a thing looks large are nearer to the thing than
those in which it looks small: they are, in fact, nearer to the
perspective which is the place where the thing is.
We can now also explain the correlation between a private space and
parts of perspective space. If there is an aspect of a given thing in a
certain private space, then we correlate the place where this aspect is
in the private space with the place where the thing is in perspective
space.
We may define "here" as the place, in perspective space, which is
occupied by our private world. Thus we can now understand what is meant
by speaking of a thing as near to or far from "here." A thing is near to
"here" if the place where it is is near to my private world. We can also
understand what is meant by saying that our private world is inside our
head; for our private world is a place in perspective space, and may be
part of the place where our head is.
It will be observed that _two_ places in perspective space are
associated with every aspect of a thing: namely, the place where the
thing is, and the place which is the perspective of which the aspect in
question forms part. Every aspect of a thing is a member of two
different classes of aspects, namely: (1) the various aspects of the
thing, of which at most one appears in any given perspective; (2) the
perspective of which the given aspect is a member, _i.e._ that in which
the thing has the given aspect. The physicist naturally classifies
aspects in the first way, the psychologist in the second. The two places
associated with a single aspect correspond to the two ways of
classifying it. We may distinguish the two places as that _at_ which,
and that _from_ which, the aspect appears. The "place at which" is the
place of the thing to which the aspect belongs; the "place from which"
is the place of the perspective to which the aspect belongs.
Let us now endeavour to state the fact that the aspect which a thing
presents at a given place is affected by the intervening medium. The
aspects of a thing in different perspectives are to be conceived as
spreading outwards from the place where the thing is, and undergoing
various changes as they get further away from this place. The laws
according to which they change cannot be stated if we only take account
of the aspects that are near the thing, but require that we should also
take account of the things that are at the places from which these
aspects appear. This empirical fact can, therefore, be interpreted in
terms of our construction.
We have now constructed a largely hypothetical picture of the world,
which contains and places the experienced facts, including those derived
from testimony. The world we have constructed can, with a certain amount
of trouble, be used to interpret the crude facts of sense, the facts of
physics, and the facts of physiology. It is therefore a world which
_may_ be actual. It fits the facts, and there is no empirical evidence
against it; it also is free from logical impossibilities. But have we
any good reason to suppose that it is real? This brings us back to our
original problem, as to the grounds for believing in the existence of
anything outside my private world. What we have derived from our
hypothetical construction is that there are no grounds _against_ the
truth of this belief, but we have not derived any positive grounds in
its favour. We will resume this inquiry by taking up again the question
of testimony and the evidence for the existence of other minds.
It must be conceded to begin with that the argument in favour of the
existence of other people's minds cannot be conclusive. A phantasm of
our dreams will appear to have a mind--a mind to be annoying, as a rule.
It will give unexpected answers, refuse to conform to our desires, and
show all those other signs of intelligence to which we are accustomed in
the acquaintances of our waking hours. And yet, when we are awake, we do
not believe that the phantasm was, like the appearances of people in
waking life, representative of a private world to which we have no
direct access. If we are to believe this of the people we meet when we
are awake, it must be on some ground short of demonstration, since it is
obviously possible that what we call waking life may be only an
unusually persistent and recurrent nightmare. It may be that our
imagination brings forth all that other people seem to say to us, all
that we read in books, all the daily, weekly, monthly, and quarterly
journals that distract our thoughts, all the advertisements of soap and
all the speeches of politicians. This _may_ be true, since it cannot be
shown to be false, yet no one can really believe it. Is there any
_logical_ ground for regarding this possibility as improbable? Or is
there nothing beyond habit and prejudice?
The minds of other people are among our data, in the very wide sense in
which we used the word at first. That is to say, when we first begin to
reflect, we find ourselves already believing in them, not because of any
argument, but because the belief is natural to us. It is, however, a
psychologically derivative belief, since it results from observation of
people's bodies; and along with other such beliefs, it does not belong
to the hardest of hard data, but becomes, under the influence of
philosophic reflection, just sufficiently questionable to make us desire
some argument connecting it with the facts of sense.
The obvious argument is, of course, derived from analogy. Other people's
bodies behave as ours do when we have certain thoughts and feelings;
hence, by analogy, it is natural to suppose that such behaviour is
connected with thoughts and feelings like our own. Someone says, "Look
out!" and we find we are on the point of being killed by a motor-car; we
therefore attribute the words we heard to the person in question having
seen the motor-car first, in which case there are existing things of
which we are not directly conscious. But this whole scene, with our
inference, may occur in a dream, in which case the inference is
generally considered to be mistaken. Is there anything to make the
argument from analogy more cogent when we are (as we think) awake?
The analogy in waking life is only to be preferred to that in dreams on
the ground of its greater extent and consistency. If a man were to dream
every night about a set of people whom he never met by day, who had
consistent characters and grew older with the lapse of years, he might,
like the man in Calderon's play, find it difficult to decide which was
the dream-world and which was the so-called "real" world. It is only the
failure of our dreams to form a consistent whole either with each other
or with waking life that makes us condemn them. Certain uniformities are
observed in waking life, while dreams seem quite erratic. The natural
hypothesis would be that demons and the spirits of the dead visit us
while we sleep; but the modern mind, as a rule, refuses to entertain
this view, though it is hard to see what could be said against it. On
the other hand, the mystic, in moments of illumination, seems to awaken
from a sleep which has filled all his mundane life: the whole world of
sense becomes phantasmal, and he sees, with the clarity and
convincingness that belongs to our morning realisation after dreams, a
world utterly different from that of our daily cares and troubles. Who
shall condemn him? Who shall justify him? Or who shall justify the
seeming solidity of the common objects among which we suppose ourselves
to live?
The hypothesis that other people have minds must, I think, be allowed to
be not susceptible of any very strong support from the analogical
argument. At the same time, it is a hypothesis which systematises a vast
body of facts and never leads to any consequences which there is reason
to think false. There is therefore nothing to be said against its truth,
and good reason to use it as a working hypothesis. When once it is
admitted, it enables us to extend our knowledge of the sensible world by
testimony, and thus leads to the system of private worlds which we
assumed in our hypothetical construction. In actual fact, whatever we
may try to think as philosophers, we cannot help believing in the minds
of other people, so that the question whether our belief is justified
has a merely speculative interest. And if it is justified, then there is
no further difficulty of principle in that vast extension of our
knowledge, beyond our own private data, which we find in science and
common sense.
This somewhat meagre conclusion must not be regarded as the whole
outcome of our long discussion. The problem of the connection of sense
with objective reality has commonly been dealt with from a standpoint
which did not carry initial doubt so far as we have carried it; most
writers, consciously or unconsciously, have assumed that the testimony
of others is to be admitted, and therefore (at least by implication)
that others have minds. Their difficulties have arisen after this
admission, from the differences in the appearance which one physical
object presents to two people at the same time, or to one person at two
times between which it cannot be supposed to have changed. Such
difficulties have made people doubtful how far objective reality could
be known by sense at all, and have made them suppose that there were
positive arguments against the view that it can be so known. Our
hypothetical construction meets these arguments, and shows that the
account of the world given by common sense and physical science can be
interpreted in a way which is logically unobjectionable, and finds a
place for all the data, both hard and soft. It is this hypothetical
construction, with its reconciliation of psychology and physics, which
is the chief outcome of our discussion. Probably the construction is
only in part necessary as an initial assumption, and can be obtained
from more slender materials by the logical methods of which we shall
have an example in the definitions of points, instants, and particles;
but I do not yet know to what lengths this diminution in our initial
assumptions can be carried.
LECTURE IV
THE WORLD OF PHYSICS AND THE WORLD OF SENSE