Again, that the real contrary to realism is _subjective_ idealism is confirmed by the history of the theory of knowledge from Descartes onwards. For the initial supposition which has originated and sustained the problem is that in knowledge the mind is, at any rate in the first instance, confined within itself. This supposition granted, it has always seemed that, while there is no difficulty in understanding the mind's acquisition of knowledge of what belongs to its own being, it is difficult, if not impossible, to understand how it can acquire knowledge of what does not belong to its own being. Further, since the physical world is ordinarily thought of as something 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.