a priori prior to or independent of experience; contrasted with ‘a posteriori’ (empirical). These two terms are primarily used to mark a distinction between (1) two modes of epistemic justification, together with derivative distinctions between (2) kinds of propositions, (3) kinds of knowledge, and (4) kinds of argument. They are also used to indicate a distinction between (5) two ways in which a concept or idea may be acquired. (1) A belief or claim is said to be justified a priori if its epistemic justification, the reason or warrant for thinking it to be true, does not depend at all on sensory or introspective or other sorts of experience; whereas if its justification does depend at least in part on such experience, it is said to be justified a posteriori or empirically. This specific distinction has to do only with the justification of the belief, and not at all with how the constituent concepts are acquired; thus it is no objection to a claim of a priori justificatory status for a particular belief that experience is required for the acquisition of some of the constituent concepts. It is clear that the relevant notion of experience includes sensory and introspective experience, as well as such things as kinesthetic experience. Equally clearly, to construe experience in the broadest possible sense of, roughly, a conscious undergoing of any sort would be to destroy the point of the distinction, since even a priori justification presumably involves some sort of conscious process of awareness. The construal that is perhaps most faithful to the traditional usage is that which construes experience as any sort of cognitive input that derives, presumably causally, from features of the actual world that may not hold in other possible worlds. Thus, e.g., such things as clairvoyance or telepathy, if they were to exist, would count as forms of experience and any knowledge resulting therefrom as a posteriori; but the intuitive apprehension of properties or numbers or other sorts of abstract entities that are the same in all possible worlds, would not. Understood in this way, the concept of a priori justification is an essentially negative concept, specifying as it does what the justification of the belief does not depend on, but saying nothing about what it does depend on. Historically, the main positive conception was that offered by proponents of rationalism (such as Plato, Descartes, Spinoza, and Leibniz), according to which a priori justification derives from the intuitive apprehension of necessary facts pertaining to universals and other abstract entities. (Although Kant is often regarded as a rationalist, his restriction of substantive a priori knowledge to the world of appearances represents a major departure from the main rationalist tradition.) In contrast, proponents of traditional empiricism, if they do not repudiate the concept of a priori justification altogether (as does Quine), typically attempt to account for such justification by appeal to linguistic or conceptual conventions. The most standard formulation of this empiricist view (a development of the view of Hume that all a priori knowledge pertains to ‘relations of ideas’) is the claim (typical of logical positivism) that all a priori knowable claims or propositions are analytic. (A rationalist would claim in opposition that at least some a priori claims or propositions are synthetic.) (2) A proposition that is the content of an a priori justified belief is often referred to as an a priori proposition (or an a priori truth). This usage is also often extended to include any proposition that is capable of being the content of such a belief, whether it actually has this status or not. (3) If, in addition to being justified a priori or a posteriori, a belief is also true and satisfies whatever further conditions may be required for it to constitute knowledge, that knowledge is derivatively characterized as a priori or a posteriori (empirical), respectively. (Though a priori justification is often regarded as by itself guaranteeing truth, this should be regarded as a further substantive thesis, not as part of the very concept of a priori justification.) Examples of knowledge that have been classically regarded as a priori in this sense are mathematical knowledge, knowledge of logical truths, and knowledge of necessary entailments and exclusions of commonsense concepts (‘Nothing can be red and green all over at the same time’, ‘If A is later than B and B is later than C, then A is later than C’); but many claims of metaphysics, ethics, and even theology have also been claimed to have this status. (4) A deductively valid argument that also satisfies the further condition that each of the premises (or sometimes one or more particularly central premises) are justified a priori is referred to as an a priori argument. This label is also sometimes applied to arguments that are claimed to have this status, even if the correctness of this claim is in question. (5) In addition to the uses just catalogued that derive from the distinction between modes of justification, the terms ‘a priori’ and ‘a posteriori’ are also employed to distinguish two ways in which a concept or idea might be acquired by an individual person. An a posteriori or empirical concept or idea is one that is derived from experience, via a process of abstraction or ostensive definition. In contrast, an a priori concept or idea is one that is not derived from experience in this way and thus presumably does not require any particular experience to be realized (though the explicit realization of such a concept might still require experience as a ‘trigger’). The main historical account of such concepts, again held mainly by rationalists, construes them as innate, either implanted in the mind by God or, in the more contemporary version of the claim held by Chomsky, Fodor, and others, resulting from evolutionary development. Concepts typically regarded as having this sort of status include the concepts of substance, causation, God, necessity, infinity, and many others. Empiricists, in contrast, typically hold that all concepts are derived from experience. See also ANALYTIC – SYNTHETIC DISTINC — TION , NECESSITY , RATIONALIS. L.B.