A Priori and A Posteriori

The terms “a priori” and “a posteriori” are used primarily to denote the foundations upon which a proposition is known. A given proposition is knowable a priori if it can be known independent of any experience other than the experience of learning the language in which the proposition is expressed, whereas a proposition that is knowable a posteriori is known on the basis of experience. For example, the proposition that all bachelors are unmarried is a priori, and the proposition that it is raining outside now is a posteriori.

The distinction between the two terms is epistemological and immediately relates to the justification for why a given item of knowledge is held. For instance, a person who knows (a priori) that “All bachelors are unmarried” need not have experienced the unmarried status of all—or indeed any—bachelors to justify this proposition. By contrast, if I know that “It is raining outside,” knowledge of this proposition must be justified by appealing to someone’s experience of the weather.

The a priori /a posteriori distinction, as is shown below, should not be confused with the similar dichotomy of the necessary and the contingent or the dichotomy of the analytic and the synthetic. Nonetheless, the a priori /a posteriori distinction is itself not without controversy. The major sticking-points historically have been how to define the concept of the “experience” on which the distinction is grounded, and whether or in what sense knowledge can indeed exist independently of all experience. The latter issue raises important questions regarding the positive, that is, actual, basis of a priori knowledge — questions which a wide range of philosophers have attempted to answer. Kant, for instance, advocated a “transcendental” form of justification involving “rational insight” that is connected to, but does not immediately arise from, empirical experience.

This article provides an initial characterization of the terms “a priori” and “a posteriori,” before illuminating the differences between the distinction and those with which it has commonly been confused. It will then review the main controversies that surround the topic and explore opposing accounts of a positive basis of a priori knowledge that seek to avoid an account exclusively reliant on pure thought for justification.