On Being and Essence

Thomas Aquinas composed this brief treatise around 1252 as a young bachelor of arts at the University of Paris, addressing fundamental questions about the nature of reality that occupied medieval philosophers and theologians. Written for his fellow Dominican students, the work tackles the relationship between what things are (essence) and the fact that they exist (being), a distinction crucial to understanding how created reality relates to God as pure existence itself.

Aquinas argues that in all created things, essence and existence are really distinct — what makes a thing the kind of thing it is differs from the act by which it actually exists. This distinction allows him to explain how creatures depend entirely on God for their being while maintaining their own genuine natures. He demonstrates that in material substances, essence includes both form and matter, while in separate substances like angels, essence is form alone. Throughout, he shows how this metaphysical framework illuminates the unique nature of God, in whom essence and existence are identical, making God the source of all being while remaining utterly transcendent.

This compact treatise became foundational to Thomistic metaphysics and remains essential reading in Catholic philosophical theology. Its influence extends far beyond medieval scholasticism, shaping centuries of reflection on the God-world relationship and the nature of finite existence. Students of Aquinas seeking to understand his mature theological synthesis must grapple with these fundamental distinctions. Those interested in the philosophical foundations of classical theism will find here the conceptual tools that enabled Aquinas to articulate how creatures can be genuinely real while remaining completely dependent on divine causality.