The way in which simplicity comes in from supposing that there really
are physical objects is easily seen. If the cat appears at one moment
in one part of the room, and at another in another part, it is natural
to suppose that it has moved from the one to the other, passing over a
series of intermediate positions. But if it is merely a set of
sense-data, it cannot have ever been in any place where I did not see
it; thus we shall have to suppose that it did not exist at all while I
was not looking, but suddenly sprang into being in a new place. If
the cat exists whether I see it or not, we can understand from our own
experience how it gets hungry between one meal and the next; but if it
does not exist when I am not seeing it, it seems odd that appetite
should grow during non-existence as fast as during existence. And if
the cat consists only of sense-data, it cannot be hungry, since no
hunger but my own can be a sense-datum to me. Thus the behaviour of
the sense-data which represent the cat to me, though it seems quite
natural when regarded as an expression of hunger, becomes utterly
inexplicable when regarded as mere movements and changes of patches of
colour, which are as incapable of hunger as a triangle is of playing
football. But the difficulty in the case of the cat is nothing compared to the
difficulty in the case of human beings. When human beings speak, that
is, when we hear certain noises which we associate with ideas, and
simultaneously see certain motions of lips and expressions of face, it
is very difficult to suppose that what we hear is not the expression
of a thought, as we know it would be if we emitted the same sounds.
Of course similar things happen in dreams, where we are mistaken as to
the existence of other people. But dreams are more or less suggested
by what we call waking life, and are capable of being more or less
accounted for on scientific principles if we assume that there really
is a physical world. Thus every principle of simplicity urges us to
adopt the natural view, that there really are objects other than
ourselves and our sense-data which have an existence not dependent
upon our perceiving them.
Of course it is not by argument that we originally come by our belief
in an independent external world. We find this belief ready in
ourselves as soon as we begin to reflect: it is what may be called an
instinctive belief. We should never have been led to question this
belief but for the fact that, at any rate in the case of sight, it
seems as if the sense-datum itself were instinctively believed to be
the independent object, whereas argument shows that the object cannot
be identical with the sense-datum. This discovery,
however, which is not at all paradoxical in the case of taste and smell and sound, and
only slightly so in the case of touch, leaves undiminished our
instinctive belief that there are objects corresponding to our
sense-data. Since this belief does not lead to any difficulties, but
on the contrary tends to simplify and systematize our account of our
experiences, there seems no good reason for rejecting it. We may
therefore admit, though with a slight doubt derived from dreams, that
the external world does really exist, and is not wholly dependent for
its existence upon our continuing to perceive it.