ousa

Ousia (Greek: οὐσία) is a philosophical term usually translated as substance or essence. In ancient Greek thought, it denotes the fundamental reality that makes a thing what it is and serves as the primary subject of predication. The noun is related to the feminine participle ousa, meaning “being” or “that which exists.”

In Aristotle’s Metaphysics, ousia is central to the study of being. Substances are the beings that exist independently and bear predicates, with primary substances being individual things (for example, a particular person or horse) and secondary substances referring to the kinds or species to which those individuals belong. Ousia is the core of what a thing is—the combination of its form and its material substrate—upon which its other properties supervene. This has been described in terms of a hylomorphic framework, where form and matter combine to realize an object’s essence.

Throughout medieval philosophy, the term ousia remained a foundational concept in discussions of ontology and metaphysics, often contrasted with accidents (properties that do not constitute the thing’s identity) and with substrate concepts such as hypokeimenon. In Christian theology, ousia acquired significant doctrinal importance through the term homoousios, meaning “of the same substance,” used to articulate the relationship between Father and Son in early ecumenical debates.

Etymology and usage: ousia derives from the verb “to be.” The present participle feminine ousa highlights the ongoing state of being. In English, ousia is typically rendered as substance or essence, and it continues to appear in historical discussions of philosophy and theology.