ONTOLOGY. The word ontology, meaning "discourse about, or study of, being," was introduced into the philosophical vocabulary in the early seventeenth century. The term was originally used as an equivalent for "metaphysics," which Aristotle, in Metaphysics 4.1, had defined precisely as the science that treats "being insofar as it is being." Thus the enterprise of ontology had a long prehistory.

Plato had considered the question of "being" (to on, ousia), which for him meant the "what" of things as a stable object of certain knowledge. Hence he thought that the term being was properly employed only of the self-identical, changeless, and hence eternal, realm of Forms—that reality, grasped by intellect alone, which is imaged in, but at the same time contrasted with, the mutable realm of "becoming." It was Aristotle, critical of this outright identification of being with the immutable and transcendent Forms, who insisted that the verb...