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.

The argument which has led us to this conclusion is doubtless less
strong than we could wish, but it is typical of many philosophical
arguments, and it is therefore worth while to consider briefly its
general character and validity. All knowledge, we find, must be built
up upon our instinctive beliefs, and if these are rejected, nothing
is left. But among our instinctive beliefs some are much stronger than
others, while many have, by habit and association, become entangled with
other beliefs, not really instinctive, but falsely supposed to be part
of what is believed instinctively.

Philosophy should show us the hierarchy of our instinctive beliefs,
beginning with those we hold most strongly, and presenting each as much
isolated and as free from irrelevant additions as possible. It should
take care to show that, in the form in which they are finally set forth,
our insti