The topic of this paper is a concept that has figured prominently in the history of philosophy, and is probably more important now than it has ever been before. It plays a crucial role in contemporary discussions of the nature of people, the nature of mental phenomena, and ontological theses like materialism (the thesis that only physical objects exist) and physicalism (the thesis that everything that exists is just what a true and complete physics would say exists). In addition, the vast majority of living philosophers begin their ontologies with a category for objects that fall under this concept. I refer, of course, to the concept of a physical object.