A Priori and A Posteriori

       Immanuel Kant argued that moral requirements are based on a standard of rationality he named the categorical imperative. Immorality thus involves a violation of the categorical imperative and is thereby irrational.

       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 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.