A PRIORI (Lat. a, from, prior, prius, that which is before, precedes), (1) a phrase used popularly of a judgment based on general considerations in the absence of particular evidence; (2) a logical term first used, apparently, by Albert of Saxony (14th century), though the theory which it denotes is as old as Aristotle. In the order of human knowledge the particular facts of experience come first and are the basis of generalized laws or causes (the Scholastic notiora nobis); but in the order of nature the latter rank first as the self-existent, fundamental truths of existence (notiora naturae). Thus to Aristotle the a priori argument is from law or cause to effect, as opposed to what we call a posteriori (posterior, subsequent, derived), from effect to cause. Since Kant the two phrases have become purely adjectival (instead of adverbial) with a technical controversial sense, closely allied to the Aristotelian, in relation to knowledge and judgments generally. A priori is applied to judgments which are regarded as independent of experience, and belonging to the essence of thought; a posteriori to those which are derived from particular observations. The distinction is analogous to that between analysis and synthesis, deduction and induction (but there may be a synthesis of a priori judgments, cf. Kant's "Synthetic Judgment a priori"). Round this distinction a rather barren controversy has raged, and almost all modern philosophers have labelled themselves either "Intuitionalist" (a priori) or "Empiricist" (a posteriori) according to the view they take of knowledge. In fact, however, the rival schools are generally arguing at cross purposes; there is a knowledge based on particulars, and also a knowledge of laws or causes. But the two work in different spheres, and are complementary. The observation of isolated particulars gives not necessity, but merely strong probability; necessity is purely intellectual or "transcendental." If the empiricist denies the intellectual element in scientific knowledge, he must not claim absolute validity for his conclusions; but he may hold against the intuitionalist that absolute laws are impossible to the human intellect. On the other hand, pure a priori knowledge can be nothing more than form without content (e.g. formal logic, the laws of thought). The simple fact at the bottom of the controversy is that in all empirical knowledge there is an intellectual element, without which there is no correlation of empirical data, and every judgment, however simple, postulates a correlation of some sort if only that between the predicate and its contradictory.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)