On Being and Essence

The central thesis is that in all created beings, essence (what a thing is) and existence (that it is) are really distinct, while in God they are identical. Aquinas argues that essence is that through which a thing is what it is, and existence is the act of being by which a thing actually exists. In creatures, essence does not include existence: a human essence can be understood without knowing whether any human exists. Only in God is essence identical with existence, because God is being itself . This position rejects the view that existence is an accident added to essence, and it rejects the view that essences are separate Platonic forms. The work is a foundational statement of Aquinas's metaphysics of being.

Aquinas distinguishes three kinds of essence. Material substances are composed of matter and form; their essence includes both, and their individuality is due to designated matter. Immaterial substances (angels) are pure forms without matter; their essence is their form, and their individuality is due to the fact that each is a distinct species, not a member of a species with multiple individuals. Accidents are forms that inhere in a substance; their essence is not a complete substance, and their definition includes their relation to the substance in which they inhere . This threefold distinction allows Aquinas to give a unified account of essence across the created order, while preserving the unique status of God as the only being whose essence is existence.

On Being and Essence was written around 1254–1256, early in Aquinas's career, for his fellow Dominicans at Paris. It is a short work, divided into a prologue and four chapters, and it was probably written as an introduction to metaphysics for students who were beginning their theological studies . The work survives in numerous manuscripts and was first printed in the late fifteenth century. The critical edition is found in the Leonine edition, and the English translation by Armand Maurer, published by the Pontifical Institute of Mediaeval Studies in 1949, is the standard version. The work's brevity and clarity have made it one of the most widely read of Aquinas's philosophical writings, and it is often used as an introduction to his metaphysics.

The essence–existence distinction is one of the foundational concepts of Aquinas's mature theology. It grounds his doctrine of God as ipsum esse subsistens (subsistent being itself), and it explains why creatures are radically dependent on God: they have existence only as received, while God is existence. The distinction also grounds his account of creation: God creates by giving existence to essences, and the creature's existence is a participation in God's being . In the Summa Theologica, the distinction is presupposed throughout, especially in the treatment of God's simplicity, creation, and the analogy of being. The early work thus provides the metaphysical vocabulary that the later theology would use, even though the later work does not repeat the argument in the same form.

Maurer's translation, published by the Pontifical Institute of Mediaeval Studies in 1949 and revised in 1968, is widely regarded as accurate and readable, and it remains the standard English version . It includes a substantial introduction that situates the work in its historical and philosophical context, and notes that help the reader understand the technical vocabulary. Its limitations are those of mid-twentieth-century Thomistic translation: it tends toward literal rendering of Latin terms, and it does not include the Latin text. More recent translations exist, including a 2017 version by John F. Wippel and others, but Maurer's remains the most widely used. Readers who want to engage the text seriously should consult the Latin alongside the translation.

The most common misunderstanding is that essence and existence are two separate things, as if a thing first has an essence and then receives existence as an added ingredient. Aquinas's point is subtler: essence and existence are really distinct, but they are not separate entities. Essence is the principle by which a thing is what it is, and existence is the act by which it is. In creatures, the two are distinct but inseparable; in God, they are identical . A related misunderstanding is that the distinction is merely conceptual, a way of analyzing things rather than a feature of reality. Aquinas insists that it is a real distinction, grounded in the structure of created being, and that it is the key to understanding the radical contingency of all things other than God.