stand by it the doctrine that whatever
exists, or at any rate whatever can be known to exist, must be in
some sense mental. This doctrine, which is very widely held among
philosophers, has several forms, and is advocated on several different
grounds. The doctrine is so widely held, and so interesting in itself,
that even the briefest survey of philosophy must give some account of
it.

Those who are unaccustomed to philosophical speculation may be inclined
to dismiss such a doctrine as obviously absurd. There is no doubt that
common sense regards tables and chairs and the sun and moon and material
objects generally as something radically different from minds and the
contents of minds, and as having an existence which might continue if
minds ceased. We think of matter as having existed long before there
were any minds, and it is hard to think of it as a mere product of
mental activity. But whether true or false, idealism is not to be
dismissed as obviously absurd.

We have seen that, even if physical objects do have an independent
existence, they must differ very widely from sense-data, and can only
have a _correspondence_ with sense-data, in the same sort of way in
which a catalogue has a correspondence with the things catalogued. Hence
common sense leaves us completely in the dark as to the true intrinsic
nature of physical objects, and if there were good reason to regard them
as mental, we could not legitimately reject this opinion merely because
it strikes us as strange. The truth about physical objects _must_ be
strange. It may be unattainable, but if any philosopher believes that
he has attained it, the fact that what he offers as the truth is strange
ought not to be made a ground of objection to his opinion.

The grounds on which idealism is advocated are generally grounds derived
from the theory of knowledge, that is to say, from a discussion of the
conditions which things must satisfy in order that we may be able to
know them. The first serious attempt to establish idealism on such
grounds was that of Bishop Berkeley. He proved first, by arguments which
were largely valid, that our sense-data cannot be supposed to have an
existence independent of us, but must be, in part at least, 'in' the
mind, in the sense that their existence would not continue if there were
no seeing or hearing or touching or smelling or tasting. So far, his
contention was almost certainly valid, even if some of his arguments
were not so.