On Being and Essence

A short treatise written in Paris in the mid-1250s, early in Aquinas's career, for his fellow Dominicans. It explains two basic terms of metaphysics. Drawing on Aristotle and Avicenna, Aquinas first distinguishes senses of 'being' and defines essence as what a thing's definition signifies. In material substances, essence includes both form and matter, and what makes one human differ from another is matter marked out by dimensions. He then shows how a nature can be considered in itself, in individual things, or as a universal in the mind, which explains genus, species and difference. The best-known chapter treats immaterial beings such as angels and the soul. Aquinas argues that we can understand what a man or a phoenix is without knowing whether it exists, so in every creature essence differs from existence (esse). Only in God are they the same: God is existence itself, and gives existence to everything else. A last chapter treats accidents.

It is the shortest route into Aquinas's metaphysics and his distinction between essence and existence, which runs through all his later thinking about God and creatures.