cause us to dream of a naval engagement. But although, in
this case, there is a physical cause for the sense-data, there is not a
physical object corresponding to the sense-data in the way in which an
actual naval battle would correspond.) There is no logical impossibility
in the supposition that the whole of life is a dream, in which we
ourselves create all the objects that come before us. But although this
is not logically impossible, there is no reason whatever to suppose that
it is true; and it is, in fact, a less simple hypothesis, viewed as a
means of accounting for the facts of our own life, than the common-sense
hypothesis that there really are objects independent of us, whose action
on us causes our sensations.

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