||The nature and extent of synthetic a priori knowledge is a central topic in philosophy. It engages consideration of the analytic/synthetic distinction and the a posteriori/a priori knowledge distinction. Analytic claims are traditionally supposed to be true by definition and synthetic claims are supposed true in virtue of the world. A priori known claims are supposed to be knowable independent of perceptual experience and a posteriori claims known on the basis of perceptual experience. The idea then that there could, even in principle, be synthetic a priori knowledge is of considerable interest. Such knowledge would, if it is possible, be about the world but knowable independent of perceptual experience. This issue arises in the context of many philosophical problems and has a long and complicated history.