A priori and a posteriori knowledge

Philosophers have distinguished between two kinds of knowledge, a priori and a posteriori. A priori knowledge is defined as knowledge which is warranted by reference to reason alone, without any reference to any external (to reason) experience at all. A more formal definition would be:

A belief is justified a priori iff its ultimate justifiers do not include sense-perception.

A posteriori knowledge is just all of the other knowledge, which is justified or warranted only after (posterior to) reference to some knowledge derived from sense perception.

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