On Being and EssenceThomas Aquinas

On Being and Essence is a short early work in which Aquinas lays out the metaphysical framework that supports his entire philosophy. He distinguishes essence (what a thing is) from existence (that it is) and argues that in every creature, essence and existence are really distinct. Only in God are they identical; God's essence is to exist.

In material substances, essence is composed of matter and form. Form makes a thing what it is (a horse, a human); matter individuates it (this horse, this human). In immaterial substances like angels, there is no matter, so each angel is its own species, individuated by its form alone. The treatise moves through the categories of substance and accident with precision, showing how the essence of a substance differs from the essence of an accident, and how both relate to the logical notion of genus and species.

The work's compressed argument contains the seeds of Aquinas's proofs for God's existence, his account of creation, and his theory of knowledge. If essence and existence are distinct in creatures, then no creature exists by its own nature; each requires a cause that confers existence. That cause must be something whose essence is identical with its existence: God, the subsistent act of being itself. On Being and Essence is the ontological foundation on which the Summa Theologica is built.