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Concept
A priori and a posteriori
Summary
A priori ("from the earlier") and a posteriori ("from the later") are Latin phrases used in philosophy to distinguish types of knowledge, justification, or argument by their reliance on experience. A priori knowledge is independent from any experience. Examples include mathematics, tautologies, and deduction from pure reason. A posteriori knowledge depends on empirical evidence. Examples include most fields of science and aspects of personal knowledge.
The terms originate from the analytic methods found in Organon, a collection of works by Aristotle. Prior analytics (a priori) is about deductive logic, which comes from definitions and first principles. Posterior analytics (a posteriori) is about inductive logic, which comes from observational evidence.
Both terms appear in Euclid's Elements and were popularized by Immanuel Kant's Critique of Pure Reason, an influential work in the history of philosophy. Both terms are primarily used as modifiers to the noun "knowledge" (i.e. "a priori knowledge"). A priori can be used to modify other nouns such as "truth". Philosophers may use apriority, apriorist, and aprioricity as nouns referring to the quality of being a priori.
Consider the proposition: "If George V reigned at least four days, then he reigned more than three days." This is something that one knows a priori because it expresses a statement that one can derive by reason alone.
Consider the proposition: "George V reigned from 1910 to 1936." This is something that (if true) one must come to know a posteriori because it expresses an empirical fact unknowable by reason alone.
Several philosophers, in reaction to Immanuel Kant, sought to explain a priori knowledge without appealing to, as Paul Boghossian explains, "a special faculty [intuition] ... that has never been described in satisfactory terms." One theory, popular among the logical positivists of the early 20th century, is what Boghossian calls the "analytic explanation of the a priori." The distinction between analytic and synthetic propositions was first introduced by Kant.
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