B. A. w. RUSSELL, Essay on the Foundations of Geometry. 399 essential to his purpose that between the a priori and empirical ; the rest of his philosophical apparatus, in which a
- form of externality ' is the central conception, may be regarded
merely as establishing and explaining this distinction. In fact, however, they seem rather to confuse the issue. Mr. Eussell tells us, to begin with, that his test of apriority will be purely logical : ' Would experience be impossible if a certain axiom or postulate were denied ? ' (p. 3). This test, he considers, excludes
- any psychological implication ' from the word a priori. But the
meaning of the test obviously depends on what is to be meant by the possibility of experience ; and Mr. Eussell seems actually to use this conception in such a way as to incur both the objections which can be urged against Kant's distinction between a priori and empirical, namely (1) that the distinction is not absolute, (2) that the a priori is confused with the object of psychological experience. (1) That the distinction is not absolute would seem to follow from the twofold nature of the test which Mr. Russell gives us for the a priori. He tries to show not only that propositions which are presupposed in any particular science are a priori a perfectly clear and simple test but also to deduce the necessity of these from the necessity for experience of a ' form of exter- i- / nality '. Necessity, as he holds to be ' shown by modern logic,' always involves a ground of necessity (p. 4). But this ground must itself either be simply categorical, or else it must itself be necessary and require a further ground. In the former case we are actually trying to deduce an a priori proposition from one that, as categorical, is merely empirical ; in the latter, which Mr. Russell seems in the end inclined to accept, we must either allow an infinite regress of necessary propositions, and thus never reach the absolutely a priori, or else we must accept the view that knowledge is circular, and shall in the end return to the proposition from which we started as empirical, as being itself the ground of necessity of the a priori, and therefore itself as much a priori as the latter. Mr. Russell seems actually to accept this latter view (pp. 57-60) a view which renders his logical criterion nugatory, since it asserts that that which is presupposed in the empirical equally and in the same sense presupposes the empirical. To put the matter shortly, to show that a ' form of externality ' is necessary for the possibility of experience, can only mean to show that it is presupposed in our actual experience. And this can never prove that no experience would be possible without such a, form, unless we assume that our actual experience is itself necessary, i.e., that no other experience is possible. (2) The only escape from this self-contradiction would seem to be by way of putting the ground of necessity in some psychological fact. And this is the subterfuge to which Mr. Russell seems actually to be driven when we find him defining his deduction from the possibility of experience as a proof that the falsehood of the judgment in
