Now Kant is here, of course, assuming the main point to be established, namely, that experience is incapable of accounting for such universality and necessity as are required for our knowledge, both ordinary and scientific. We have already considered this assumption,[227] and have also anticipated misunderstanding by noting the important qualifications to which, from Kant’s new Critical standpoint, the terms ‘necessity’ and ‘universality’ become subject.[228] The very specific meaning in which Kant employs the term _a priori_ must likewise be borne in mind. Though negatively the _a priori_ is independent of experience, positively it originates in our human reason. The necessity and universality which differentiate the _a priori_ distinguish it only from the humanly accidental. The _a priori_ has no absolute validity. From a metaphysical standpoint, it is itself contingent. As already stated,[229] all truth is for Kant merely _de facto_. The necessary is not that which cannot be conceived to be otherwise, nor is it the unconditioned. Our reason legislates only for the world of appearance. But as yet Kant gives no hint of this revolutionary reinterpretation of the rationalist criteria. One of the chief unfortunate consequences of the employment in this _Introduction_ of the analytic method of the _Prolegomena_ is that it tends to mislead the reader by seeming to commit Kant to a logical _a priori_ of the Leibnizian type. =To show that, if experience is to be possible, [pure a priori propositions] are indispensable, and so to prove their existence a priori.=[230]--At first sight Kant would seem to be here referring to the alternative synthetic method of procedure, _i.e._ to the _transcendental_ proof of the _a priori_. The next sentence shows, however, that neither in intention nor in fact is that really so. He argues only that _a priori_ principles, such as the principle of causality, are necessary in order to give “certainty” to our experience; such a principle must be postulated if inductive inference is to be valid. Experience could have no [scientific] certainty, “if all rules according to which it proceeds were themselves in turn empirical, and therefore contingent. They could hardly be regarded as first principles.” There is no attempt here to prove that empirical knowledge _as such_ necessarily involves the _a priori_. Also the method of argument, though it seeks to establish the _necessity_ of the _a priori_, is not transcendental or Critical in character. It is merely a repetition of the kind of argument which both Hume and Leibniz had already directed against the sensationalist position.[231] Very strangely, considering that these sentences have been added in the second edition, and therefore subsequent to the writing of the objective deduction, Kant gives no indication of the deeper problem to which he finally penetrated. The explanation is, probably, that to do so would have involved the recasting of the entire _Introduction_.