thing which does not belong to the mind's own being, the problem
has always been not 'How is it possible to know anything?' but 'How is
it possible to know a particular kind of reality, viz. the physical
world?' Moreover, in consequence of the initial supposition, any
answer to this question has always presupposed that our apprehension
of the physical world is indirect. Since _ex hypothesi_ the mind is
confined within itself, it can only apprehend a reality independent of
it through something within the mind which 'represents' or 'copies'
the reality; and it is perhaps Hume's chief merit that he showed that
no such solution is possible, or, in other words, that, on the given
supposition, knowledge of the physical world is impossible.

Now the essential weakness of this line of thought lies in the initial
supposition that the mind can only apprehend what belongs to its own
being. It is as much a fact of our experience that we directly
apprehend bodies in space, as that we directly apprehend our feelings
and sensations. And, as has already been shown,[5] what is spatial
cannot be thought to belong to the mind's own being on the ground that
it is relative to perception. Further, if it is legitimate to ask,
'How can we apprehend what does not belong to our being?' it is
equally legitimate to ask, 'How can we apprehend what does belong to
our own being?' It is wholly arbitrary to limit the question to the
one kind of reality. If a question is to be put at all, it should take
the form, 'How is it possible to apprehend anything?' But this
question has only to be put to be discarded. For it amounts to a
demand to _explain_ knowledge; and any answer to it would involve the
derivation of knowledge from what was not knowledge, a task which must
be as impossible as the derivation of space from time or of colour
from sound. Knowledge is _sui generis_, and, as such, cannot be
explained.[6]

[5] Cf. pp. 89-91.

[6] This assertion, being self-evident, admits of no direct
proof. A 'proof' can only take the form of showing that
any supposed 'derivation' or 'explanation' of knowledge
presupposes knowledge in that from which it derives it.
Professor Cook Wilson has pointed out that we must understand
what knowing is in order to explain anything at all, so
that any proposed explanation of knowing would necessarily
presuppose that we understood what knowing is. For the
general doctrine, cf. p. 245.