A priori knowledge is independent of experience. You know it before you experience it. It comes from reason alone. All bachelors are unmarried. That is true by definition. You do not need to interview bachelors. The truth is in the meaning of the words. Two plus two equals four. That is true regardless of what you observe.

The term is Latin for from the former. It contrasts with a posteriori knowledge, which depends on experience. A priori knowledge is necessary. It cannot be otherwise. It is universal. It does not vary from person to person or place to place. Mathematics and logic are the home of a priori knowledge. They are built on definitions and rules, not observations.

Examples of a priori knowledge

• All bachelors are unmarried.
• Two plus two equals four.
• If A is greater than B and B is greater than C, then A is greater than C.
• A square has four sides.
• Nothing can be both true and false at the same time.

A priori knowledge is certain. It does not need verification. It cannot be refuted by experience. You will never find a married bachelor. You will never find a square with five sides. The truths are built into the concepts. The debate is whether a priori knowledge is about the world or about our concepts. Immanuel Kant argued that some a priori knowledge is synthetic, meaning it tells us something about the world. Others argue that a priori knowledge is analytic, true by definition but not informative about reality. The debate continues. But the concept is clear. Some knowledge comes before experience. It is the knowledge of reason.