Ousia

A noun made from 'to be'

Ousia (οὐσία) is the Greek word for being, substance, or essence — and it is, quite literally, the act of being turned into a noun, formed from ousa, the participle of the verb einai, 'to be'. In everyday Greek it could even mean one's 'substance' in the sense of property or estate. Philosophy seized on it to name what a thing most truly and fundamentally is.

The being of the Forms

For Plato, true ousia belongs to the Forms — the real, changeless beings, as against the fleeting things of sense that only ever come to be and pass away. To ask after the ousia of justice or of beauty is to ask what it really is, its essential nature, beneath all its shifting appearances.

Aristotle's substance

Aristotle made ousia the first and central category of being. He distinguished primary substance — the individual thing, this man or this horse — from secondary substance, the species and genus to which it belongs, and analyzed each substance into matter and form. The essence of a thing, what it is to be that thing, he named by a famous phrase: 'the what-it-was-to-be'.

Latin and theology

Latin rendered ousia as both substantia and essentia, the roots of our 'substance' and 'essence'. The word later became the hinge of Christian doctrine, in the declaration that the Son is homoousios — 'of one substance' — with the Father, the great term fixed at the Council of Nicaea.