a priori and a posteriori
The distinction between the a priori and the a posteriori has always been an epistemological one; that is to say , it has always had something to do with knowledge. The terms "a priori" and "a posteriori" are Scholastic terms which have their origin in certain ideas of Aristotle; but their use has been considerably extended the course in the course of history, and their present use stems from the meaning given to them by Kant. The terms literally mean "from what is prior" and "from what is posterior." According to Aristotle, A is prior to B in nature if and only if B could not exist without A; A is prior to B in knowledge if and only if we cannot know B without knowing A. It is possible for these two senses of "prior" to have an application in common; substance, for example, is prior to other things in both of these senses and in others. It follows to know something from what is prior is to know what is, in some sense, it cause. Aristotle believed that it is possible to demonstrate a causal relationship by means of a syllogism in which the term for the cause is the middle term. Hence, to know something in terms of what is prior is to know it in terms of a demonstrable causal relationship. To know something from what is posterior, on the other hand, can involve no such demonstration, since the knowledge will be inductive in form.
The transition to Kant's conception of the matter is evident in Leibniz. According to the latter, to know reality a posteriori is a know it from what is actually found in the world, that is, by the senses, by the effects of reality in experience; to know reality a priori is to know it "by exposing the cause or the possible generation of the definite thing." It is also possible to speak of a priori proofs. As a general consequence of this, Leibniz could distinguish between "truths a posteriori, or of fact," and "truths a priori, or of reason"; for a priori truths can be demonstrated in terms of their being based on identical propositions, while a posteriori truths can be seen to be true only from experience. thus the distinction between the a posteriori and the a priori comes to be a distinction between what is derived from experience and what is not, whether or not the notion of the a priori also has the notion of demonstration in terms of cause or reason associated with it. Such is the distinction in Kant, and it has remained roughly the same ever since. Since in Kant there is no simple opposition between sense experience and reason (there being also the understanding), it is not possible to express the distinction he laid down as one between what is derived from experience and what is derived from reason.
The distinction, then, is roughly equivalent to that between the empirical and the non-empirical. Kant also connected it with the distinction between the necessary and the contingent, a priori truths being necessary and a posteriori truths contingent. But to assume without further argument that the two distinctions coincide in their application is to assume too much. The same is true of the distinction between the analytic and the synthetic; this too cannot be assimilated without argument to that between the a priori and a posteriori. Whether or not these distinctions coincide in their applications, they certainly cannot have the same meaning. The distinction between the a priori and a posterioriis an epistemological one; it is certainly not evident that the others are.