a priori definition
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. 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. 1 from cause to effect or from a generalization to particular instances; deductive or deductively. 2  based on theory, logic, fixed rules or forms, etc. instead of on experience or experiment.  before examination or analysis. [Latin, From the cause to the effect.] This phrase refers to a type of reasoning that examines given general principles to discover what particular facts or real-life observations can be derived from them. Another name for this method is deductive reasoning. The a priori/a posteriori distinction is sometimes applied to things other than ways of knowing, for instance, to propositions and arguments. An a priori proposition is one that is knowable a priori and an a priori argument is one the premises of which are a priori propositions. Correspondingly, an a posteriori proposition is knowable a posteriori, while an a posteriori argument is one the premises of which are a posteriori propositions. Although the use of a priori to distinguish knowledge such as that which we have in mathematics is comparatively recent, the interest of philosophers in that kind of knowledge is almost as old as philosophy itself. In the beginning was quantum mechanics: cosmologists take a chance on a quantum universe. Inspiration for Cassam's approach is drawn from Kant's account of synthetic a priori knowledge, and is it is also influenced by Dray, Nozick and Peacocke (Chapter 1). Quassim Cassam: The Possibility of Knowledge. The Latin phrases a priori (“from what is before”) and a posteriori (“from what is after”) were used in philosophy originally to distinguish between arguments from causes and arguments from effects. The first recorded occurrence of the phrases is in the writings of the 14th-century logician Albert of Saxony. A priori knowledge. A priori knowledge, in Western philosophy since the time of Immanuel Kant, knowledge that is independent of all particular experiences, as opposed to a posteriori knowledge, which derives from experience.
One that is knowable a priori and an a priori argument is one the premises of which are a priori propositions.