he believed he had found the key to the ultimate understanding of the world of experience.
He regarded as real, genuine knowledge only such propositions as possessed absolute validity, i.e. were known to be true anywhere and at any time. These propositions must be valid a priori i.e. they cannot be based on experience, because a proposition which expresses a fact of experience is, on account of its very definition, valid for this particular fact only, and we cannot know whether it will be true also for facts at other times or in other places as long as we have no experience of them. Kant did well to insist that the term a priori must not be understood psychologically, but logically: that is to say, a judgment a priori is not one that is produced in the mind without any previous experience (this, evidently never happens, and if it did happen, the judgment might not be valid at all) but it is one, whose truth is not based on experience; it would not come into existence without experience, but does not derive its validity from it. A judgment a posteriori, on the contrary, owes its validity to experience, simply because it is nothing but the expression of a fact of experience. This distinction between the psychological and the logical a priori, between the genesis and the validity of a proposition, had not been made with sufficient clarity by Kant's predecessors, especially Locke and Hume, but it is very helpful to avoid confusion. In any discussion of the validity of propositions it is convenient to use Kant's terminology, because with its help it is easy to express the various possible opinions and to state one's own case.
I need only mention the well known distinction between synthetic and analytic judgments. The former are propositions, which, if they are true, actually contain and convey some knowledge about the world; the latter are, what we have called tautologies mere empty forms which do not impart any information about reality; (Kant saw clearly, of course, that) all analytic propositions must be a priori: the validity of tautology is quite independent of experience, as it rests on nothing but the definitions of the concepts occurring in it; if I have defined a planet, for instance, as a celestial body moving around the sun, the validity of the proposition "all planets move around the sun" is certain, it need not and cannot be established by experience, experience cannot disprove it, for if I find that a certain celestial body does not move around the sun, I cannot call it a planet, because
