A Priori / A Posteriori

Epistemology is the study of knowledge. Within the study of knowledge are two categories of knowledge—a priori and a posteriori.

A priori knowledge is knowledge that is not based on observation of the physical world. The term a priori comes from two Latin words. A means “from” and priori means “that which comes before.” Thus, a priori knowledge is knowledge that exists in the mind before any experience with or observation of the physical world. The classic example of an a priori truth is mathematics which can be known and verified apart from experience.

A posteriori knowledge, on the other hand, is knowledge that comes directly from observation of the physical world. The term a posteriori means “from what comes later” and, thus, refers to knowledge that comes as a result of experiencing the physical world.

Great debate has existed throughout the history of philosophy concerning whether our knowledge is primarily a priori or a posteriori. Rene Descartes was a big proponent of a priori knowledge while others like John Locke and David Hume stressed that all of our knowledge comes from experience. In fact, Locke argued that our minds are a tabula rasa or “blank slate” at birth. It is only through experience oft the world that we gain knowledge.