essentia

Essentia is the Latin term for essence, used in philosophy to denote the defining nature of a thing—the set of properties that make something what it is, and without which it would not be that kind of thing. An essence is contrasted with accidental or contingent properties, which may change without altering the object's basic kind.

Historically, Aristotle treated form as the principle that organizes matter, giving a thing its identity. In later scholastic philosophy, the term essentia was used to distinguish what a thing is (its essence) from that it exists (its esse). This distinction underpinned essentialist accounts of natural kinds, where entities were thought to possess a fixed set of essential properties that define their kind.

In contemporary metaphysics, debates persist over whether essences are mind-independent features of reality or constructs of language and thought. Some philosophers defend essentialism for natural kinds, mathematics, or necessary properties; others critique it through nominalism or anti-essentialism, arguing that many properties are not essential to membership in a kind or that kinds are better understood as cluster concepts rather than fixed essences.

The concept remains central to discussions of classification, science, and ontology. It informs questions about what it means for a thing to be that thing and which properties are indispensable to its identity, shaping arguments in areas ranging from metaphysics to philosophy of science.