This essay is an exploration of the ontological landscape of ordinary discourse and thought. Most philosophers would concede that there is an ordinary, commonsense, or “folk” conceptual scheme, and that this scheme has certain ontological presuppositions. Foremost among these is the idea that there are enduring things, or individual substances, continuants such as people, rocks, flowers, and houses. Other kinds of entities which common sense appears to recognize are events, places, times, properties, and collections, as well as surfaces, edges, shadows, and holes. Any ontologist must begin as a point of reference with a consideration of this folk or commonsense ontology, even if in the end he revises it in some way. At least since the time of Aristotle, philosophers have tried to organize and relate entities of the kinds which belong to the commonsense ontology, kinds which Aristotle called categories.

One of our primary aims is to analyze the ordinary or commonsense concept of an individual substance, and the other is to characterize the possible extension of this concept. These analytical enterprises do not involve any commitment to the existence of an individual substance so conceived. Our analysis of substance will be carried out in terms of a broad theory of ontological categories which covers both commonsense categories of the sort just referred to and categories of a more theoretical sort, which are scientific, mathematical, or philosophical in origin.