But this is only to repeat that such forms of unity as are necessary to self-consciousness must be realised in all synthesis. It is no sufficient proof that those forms of relation coincide with the categories. As we shall find in considering the deduction of the second edition, Kant to some extent came to recognise the existence of this gap in his argument and sought to supply the missing steps. But his method of so doing still ultimately consists in an appeal to the results of the metaphysical deduction, and therefore rests upon his untenable belief in the adequacy of formal logic. It fails to obviate the objection in any satisfactory manner. As regards the negative aspect of the conclusion reached--that the validity of the categories is established only for appearances--Kant maintains that this is a necessary corollary of their validity being _a priori_. That things in themselves must conform to the conditions demanded by the nature of our self-consciousness is altogether impossible of proof. Even granting, what is indeed quite possible, that things in themselves embody the pure forms of understanding, we still cannot have any ground for maintaining that they must do so of necessity and will be found to do so universally. For even if we could directly experience things in themselves, and apprehend them as conforming to the categories, such conformity would still be known only as contingent. But when it is recognised that nature consists for us of nothing but appearances, existing only in the mode in which they are experienced, and therefore as necessarily conforming to the conditions under which experience is alone possible, the paradoxical aspect of the apriority ascribed to the categories at once vanishes. Proof of their _a priori_ validity presupposes the phenomenal character of the objects to which they apply. They can be proved to be universal and necessarily valid of objects only in so far as it can be shown that they have antecedently conditioned and constituted them. The sole sufficient reason for asserting them to be universally valid throughout experience is that they are indispensably necessary for rendering it possible.[909] The transcendental method of proof, _i.e._ proof by reference to the very possibility of experience, is for this reason, as Kant so justly emphasises, the sole type of argument capable of fulfilling the demands which have to be met. It presupposes, and itself enforces, the truth of the fundamental Critical distinction between appearances and things in themselves.