Ousia (οὐσία) — A central term of metaphysics, rendered into Latin as substantia (substance) and also as “being” or “essence.” Etymologically, ousia derives from the feminine participle of the verb einai (“to be”), so that it literally designates “that which is” — whatever gives something its own, independent existence. Its canonical formulation is found in Aristotle. In the Categories he distinguishes primary substance — the concrete individual, such as Socrates or this horse, that of which everything else is predicated and which is itself predicated of no further subject — from secondary substance, the species and genera (human, animal) under which the individual falls. In the Metaphysics, especially Book VII (Zeta), Aristotle deepens the inquiry and tends to identify ousia not with the sensible composite but with form (eidos or morphē), the intelligible principle that makes a thing what it is. Ousia thus stands opposed to mere accidents (symbebēkota) — quality, quantity, relation — which exist only in something else, and is closely tied to the hypokeimenon, the substrate or subject that persists through change. The question of ousia — what exists in a primary and independent manner? — runs through the whole of Western metaphysics and re-emerges, in a theological key, in the Trinitarian controversies, where the term comes to name the common nature shared by the divine persons (homoousios).
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