The argument of Kant’s _Introduction_ follows the above sequence. It starts by defining the problem of metaphysical knowledge _a priori_, and through it leads up to the logical problem of the _a priori_ synthetic judgment. In respect of time all knowledge begins _with_ experience. But it does not therefore follow that it all arises _from_ experience. Our experience may be a compound of that which we receive through impressions, and of that which pure reason supplies from itself.[146] The question as to whether or not any such _a priori_ actually exists, is one that can be answered only after further enquiry. The two inseparable criteria of the _a priori_ are necessity and universality. That neither can be imparted to a proposition by experience was Kant’s confirmed and unquestioned belief. He inherited this view both from Leibniz and from Hume. It is one of the presuppositions of his argument. Experience can reveal only co-existence or sequence. It enables us only to assert that so far as we have hitherto observed, there is no exception to this or that rule. A generalisation, based on observation, can never possess a wider universality than the limited experience for which it stands. If, therefore, necessary and universal judgments can anywhere be found in our knowledge, the existence of an _a priori_ that originates independently of experience is _ipso facto_ demonstrated.[147] The contrast between empirical and _a priori_ judgments, as formulated from the dogmatic standpoint, is the most significant and striking fact in the whole range of human knowledge. _A priori_ judgments claim absolute necessity. They allow of no possible exception. They are valid not only for us, but also for all conceivable beings, however different the specific conditions of their existence, whether they live on the planet Mars or in some infinitely remote region of stellar space, and no matter how diversely their bodily senses may be organised. Through these judgments a creature five feet high, and correspondingly limited by temporal conditions, legislates for all existence and for all time. Empirical judgments, on the other hand, possess only a hypothetical certainty. We recognise that they may be overturned through some addition to our present experience, and that they may not hold for beings on other planets or for beings with senses differently constituted. Whereas the opposite of a rational judgment is not even conceivable, the opposite of an empirical judgment is always possible. The one depends upon the inherent and inalienable nature of our thinking; the other is bound up with the contingent material of sense. The one claims absolute or metaphysical truth: the other is a merely tentative _résumé_ of a limited experience.