, are found to have no validity whatever.
Hence his grounds in favour of idealism may be dismissed. It remains to
see whether there are any other grounds.

It is often said, as though it were a self-evident truism, that we
cannot know that anything exists which we do not know. It is inferred
that whatever can in any way be relevant to our experience must be at
least capable of being known by us; whence it follows that if matter
were essentially something with which we could not become acquainted,
matter would be something which we could not know to exist, and which
could have for us no importance whatever. It is generally also implied,
for reasons which remain obscure, that what can have no importance for
us cannot be real, and that therefore matter, if it is not composed of
minds or of mental ideas, is impossible and a mere chimaera.

To go into this argument fully at our present stage would be impossible,
since it raises points requiring a considerable preliminary discussion;
but certain reasons for rejecting the argument may be noticed at
once. To begin at the end: there is no reason why what cannot have any
_practical_ importance for us should not be real. It is true that,
if _theoretical_ importance is included, everything real is of _some_
importance to us, since, as persons desirous of knowing the truth about
the universe, we have some interest in everything that the universe
contains. But if this sort of interest is included, it is not the case
that matter has no importance for us, provided it exists even if we
cannot know that it exists. We can, obviously, suspect that it may
exist, and wonder whether it does; hence it is connected with our desire
for knowledge, and has the importance of either satisfying or thwarting
this desire.

Again, it is by no means a truism, and is in fact false, that we cannot
know that anything exists which we do not know. The word 'know' is here
used in two different senses. (1) In its first use it is applicable to
the sort of knowledge which is opposed to error, the sense in which
what we know is _true_, the sense which applies to our beliefs and
convictions, i.e. to what are called _judgements_. In this sense of the
word we know _that_ something is the case. This sort of knowledge may
be described as knowledge of _truths_. (2) In the second use of the word
'know' above, the word applies to our knowledge of _things_, which we
may call _acquaintance_. This is the sense in which we