priori

A priori is a Latin phrase meaning "from the former" or "from the earlier." In philosophy, the term denotes knowledge, justification, or arguments that are independent of empirical observation, derived through reason alone. It is contrasted with a posteriori knowledge, which depends on experience. The distinction is central in epistemology and has been a major topic of discussion since ancient and modern philosophy.

Immanuel Kant popularized the term synthetic a priori, referring to propositions that are informative about the world yet knowable independently of particular experiences, such as foundational principles of mathematics and the structures of understanding. Analytic a priori propositions, such as "all bachelors are unmarried," are considered true by virtue of the meanings of terms.

In statistics and probability theory, the phrase appears in the expression a priori probability or a priori distribution, denoting a prior belief about a parameter before observing data. Bayesian inference uses these priors to combine prior beliefs with evidence from data to form posterior beliefs.

In everyday language, a priori can describe reasoning that proceeds from theoretical deduction rather than observation, often implying assumptions accepted on principle rather than empirical evidence. The term is usually written as "a priori" in English; occasionally it is shortened to "priori" in scholarly contexts, though the standard form is "a priori." The expression should not be confused with priory, a separate word referring to a monastery or its buildings.