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