The concepts of essence and existence are among the most foundational topics in metaphysics, the branch of philosophy concerned with the nature of reality and being. These concepts have been explored extensively by philosophers from Aristotle to Aquinas, and later by Descartes, Leibniz, Kant, and Heidegger. Their distinctions and interrelations provide crucial insights into how beings are defined and how they come to be.

Essence: What Something Is

Essence refers to the whatness or quiddity of a thing—what makes it what it is, as opposed to something else. The term comes from the Latin essentia, itself derived from the Greek ousia (being).

• For Aristotle, essence is what is expressed by the definition of a thing. In Metaphysics Book Zeta, he writes that the essence is “what it is to be” (to ti ēn einai).

• In scholastic terminology, essence is sometimes called the formal cause—the form or nature that specifies a being. For example, the essence of a triangle is “a three-sided plane figure.”

• Essence is distinct from accidents, which are properties that a thing can lose or gain without ceasing to be what it is. For example, the color of a horse is an accidental property, but being an animal is essential.

Thus, essence is the necessary content without which an entity would not be the kind of thing it is.