In ancient and modern Western thought, the problem of the nature of categories has been inseparable from arguments about the nature of selfhood; about how knowledge is organised; about how power should be distributed; and about how history should be understood. For Plato, Forms belonging to a timeless order of being played the role of categories or fundamental concepts; for Aristotle categories were immanent in things; for Kant they were a priori logical structures of our consciousness; and for Hegel they were dynamic, dialectical inter-related ideas. In Categories, O'Sullivan shows how these answers have gone forward into the contemporary era, and identifies three key schools of thought that have developed since Hegel in particular. He explains modern thought as a tension between a desire for a single dominant perspective, whether scientific or phenomenological; a belief in irretrievable fragmentation; and an effort to find a middle ground.