A priori

3. Prominent explanations of the a priori

Philosophers have long sought an account of the defining feature of truths that humans can know a priori. One result is a variety of accounts of the a priori in circulation. Psychologism about the a priori, advanced initially but later opposed by Husserl, claims that a true proposition is knowable a priori by humans if and only if our psychological constitution precludes our regarding that proposition as false. Linguisticism about the a priori, endorsed by A.J. Ayer and various other twentieth-century empiricists, states that a true proposition is knowable a priori if and only if our denying that proposition would violate rules of coherent language-use; this view denies the existence of synthetic a priori truths. Pragmatism about the a priori, advanced by C.I. Lewis, claims that a true proposition is knowable a priori by a person if and only if it describes their pragmatically guided intention to use a certain conceptual scheme of classification for the organizing of experiences. Lewis argued that pragmatic considerations regarding what suits one’s needs guide the way in which one formulates a conceptual scheme. A different view, supported by Roderick Chisholm and many others, affirms that a true proposition is knowable a priori by us if and only if our understanding that proposition is all the evidence we need to see that the proposition in question is true. Yet another view about the a priori is suggested by the later writings of Wittgenstein: A proposition is knowable a priori by us if and only if our ‘forms of life’ (that is, human nature as determined by our biology and cultural history) preclude the intelligibility for us of the denial of that proposition. (Wittgenstein did not offer a detailed account of ‘forms of life’ or of their role in determining what is a priori.) These are the most influential, but not the only, accounts of the a priori in circulation.

A theory of a priori knowledge should identify the strengths and weakness of the aforementioned accounts of the a priori. It should also identify the feature of a priori justification that requires limitation of the set of propositions knowable a priori to the distinctive kind of propositions specified by that theory. Such a theory must avoid confusing the notion of what is a priori with the notions of what is necessarily true, what is analytically true, what is innate, and what is certain. It must also draw a clear distinction between what is a priori and what is a posteriori.