The terms "a priori" and "a posteriori" are used in philosophy (epistemology) to distinguish two types of knowledge, justifications or arguments. A priori knowledge or justification is independent of experience (for example 'All bachelors are unmarried'); a posteriori knowledge or justification is dependent on experience or empirical evidence (for example 'Some bachelors are very happy'). A priori justification makes reference to experience; but the issue concerns how one knows the proposition or claim in question—what justifies or grounds one's belief in it.

Galen Strawson wrote that an a priori argument is one of which "you can see that it is true just lying on your couch. You don't have to get up off your couch and go outside and examine the way things are in the physical world. You don't have to do any science." There are many points of view on these two types of assertion, and their relationship is one of the oldest problems in modern philosophy.

See also the related distinctions: deductive/inductive, analytic/synthetic, necessary/contingent. The terms "a priori" and "a posteriori" are used in philosophy to distinguish two different types of knowledge, justification, or argument: 'a priori knowledge' is known independently of experience, and 'a posteriori knowledge' is proven through experience. Thus, they are primarily used as adjectives to modify the noun "knowledge", or taken to be compound nouns that refer to types of knowledge (for example, "a priori knowledge").

However, "a priori" is sometimes used as an adjective to modify other nouns, such as "truth". Additionally, philosophers often modify this use. For example, "apriority" and "aprioricity" are sometimes used as nouns to refer (approximately) to the quality of being a priori.

Examples of proposed candidates of a priori knowledge include "2+5=7", the propositions of Euclidean geometry, and "all bachelors are unmarried." Examples of proposed candidates of a posteriori reasoning include "protons are made of quarks" and "Hitler died in 1945."

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