Kant’s position, when thus stated, differs from that of Leibniz only in its clearer grasp of the issues and difficulties involved, and consequently in the more subtle, pertinacious, and thoroughgoing character of the argument by which it is established. Its revolutionary character first appears when Kant further argues, in extension of the teaching of Hume, that the formal, relational elements are of a _synthetic_ nature. The significance and scope of this conclusion can hardly be exaggerated. No other Kantian tenet is of more fundamental importance.[24] With it the main consequences of Kant’s Critical teaching are indissolubly bound up. _As the principles which lie at the basis of our knowledge are synthetic, they have no intrinsic necessity, and cannot possess the absolute authority ascribed to them by the rationalists._ They are prescribed to human reason, but cannot be shown to be inherently rational in any usual sense of that highly ambiguous term. They can be established only as brute conditions, verifiable in fact though not demonstrable in pure theory (if there be any such thing), of our actual experience. They are conditions of _sense_-experience, and that means of our knowledge of appearances, never legitimately applicable in the deciphering of ultimate reality. They are valid within the realm of experience, useless for the construction of a metaphysical theory of things in themselves. This conclusion is reinforced when we recognise that human experience, even in its fundamental features (_e.g._ the temporal and the spatial), might conceivably be altogether different from what it actually is, and that its presuppositions are always, therefore, of the same contingent character. Even the universality and necessity which Kant claims to have established for his _a priori_ principles are of this nature. Their necessity is always for us extrinsic; they can be postulated only if, and so long as, we are assuming the occurrence of human sense-experience. Thus Kant is a rationalist of a new and unique type. He believes in, and emphasises the importance of, the _a priori_. With it alone, he contends, is the _Critique_ competent to deal. But it is an _a priori_ which cannot be shown to be more than relative. It does, indeed, enable us to conceive the known as relative, and to entertain in thought the possibility of an Absolute; but this it can do without itself possessing independent validity. For though the proof of the _a priori_ is not empirical in the sense of being inductive, neither is it logical in the sense of being deduced from necessities of thought. Its “transcendental” proof can be executed only so long as experience is granted as actual; and so long as the fundamental characteristics of this experience are kept in view.