“Analytic” truths are true by definition and tell us nothing about the world. “Synthetic” truths are about the world, but they can turn out to be false. Along with this distinction is the a priori/a posteriori distinction: a priori knowledge is known without, or before, experience, whereas a posteriori knowledge can only be known after, or as a result of, experience.

Empiricist philosophers traditionally hold that there are no a priori synthetic truths, and they have tended to assume that what is analytic is also a priori, and what is synthetic is a posteriori.

Lewis’ main philosophical tool, in accounting for both ordinary experience and scientific knowledge, was to distinguish between the a priori and what he called “the given.” Quite simply, he thought that our knowledge and experience was the result of the interplay between the a priori and the given. There is something “brute” in our experience that we have no control over, but we make sense of it by projecting a priori principles and categories onto it.