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In classical Greek philosophy, ousia (a noun derived from the present participle of the Greek verb "to be") most often expresses one or another of four closely connected concepts: (1) what something is in itself, its being or essence; (2) an entity which is what it is, at least with respect to essential attributes, on its own and without dependence on any more fundamental entity of another type outside itself (in Plato's middle dialogues, the forms; for Aristotle, substance; for the Stoics, the material substrate); (3) for Plato, being as opposed to becoming; and (4) for the Stoics in some instances, existence as opposed to nonexistence. Depending on the context, ousia may be translated as "being," "essence," "reality," or "substance."

Employed in ordinary Greek to speak of a person's wealth and possessions, the word ousia was put to philosophical use by Plato in his early dialogue Euthyphro to state a requirement on definitions. Asked what piety is, Euthyphro answers that it is what is loved by all the gods. Socrates responds with a clear statement of concept (1), saying that Euthyphro has mentioned merely something that qualifies piety externally and has failed to give the ousia of piety (11a4b1), what it is in itself that leads the gods to love it.

The transition from concept (1) to concept (2) occurs most clearly in the Phaedo, a dialogue of Plato's middle period. There the character Socrates introduces several forms, including the just itself and the beautiful itself (65d48), and speaks of them as the ousia of other things (65d13), in the sense that other things become just or beautiful, for example, only by participation in, or dependence on, the corresponding form (101c34). Each such form is an ousia according to concept (2) (76d9, cf. 77a2), a being or reality (78d1) that is always the same and unchanging (78d17), an object of thought rather than sensation.

In the Republic a similar picture obtains, but there the character Socrates speaks of the forms collectively as ousia, with the exception of the form of the good (VI, 509b89), and contrasts this invariant, unqualified, and cognitively reliable being first with the many sensible things, which can appear, for example, beautiful in one respect but ugly in another (V, 479c7, cf. 479b6d1), and then with the collective becoming and decaying of these sensibles (VI, 485b21). This use of the word ousia to express concept (3), being as opposed to becoming, is frequent in book VII, where the study of the mathematical sciences serves to lead the prospective philosopher-rulers to turn away from becoming and toward being (VII, 525b5, cf. 525c6, 526e6, 534a3). This strong distinction in the Republic between being and becoming has been questioned by some scholars. In any case, it is considerably attenuated in some of Plato's later dialogues, including the Philebus, where the character Socrates asserts "Every process of generation takes place for the sake of some particular being [ousias tinas hekastēs ]" (54c23).

In the Categories, Aristotle uses the word ousia occasionally in the concept (1) sense of essence (e.g., at 1a12), but at the center of the discussion in the Categories is concept (2), and ousia in this sense becomes a technical term rendered by most translators as "substance." Moving even further from the view of the Republic than Plato does in his later dialogues, Aristotle argues that ousia in sense (2) belongs primarily and most of all to sensible entities like a particular human or a particular horse (2a1114), since these "primary substances" (2a35) are substrates, or ontological subjects, not only of their own essential attributes but also, differently, of inherents from other categories, such as a certain quality or a certain quantity, that happen to be "in" them at one time or another (2a34b5). He concludes that everything else under discussion in the Categories, including the species and genera of primary substances (called "secondary substances" at 2a14) as well as all the inherents in other categories, depend on primary substances for their being, in the sense that without primary substances, none of the others could be (2b56). (For an even stronger claim that all depends on substance, the focal or referential theory of the meaning of "being" [Gk. "to on," the participle], see Metaphysics, IV, 1003b510; cf. Devereux, pp. 220, 232.)

Aristotle's other extended discussion of ousia (Metaphysics, VII, VIII) accepts the view of the Categories that particular animals and plants fall under ousiai in sense (2) (VII, 1028b810). But book VII, having brought in the distinction between matter and form introduced in the Physics (190b1191a22) to explain the coming-to-be and passing-away of particular sensible substances, subsequently regarded as composites of matter and form, says that such composite sensible substances are "posterior" to both matter and form (1029a3032). It then argues at length for the thesis that form is primary substance (1037a57 and 1037a2730, cf. 1032b12). This thesis raises two important questions. How does the thesis fit with Aristotle's position in the Categories that entities like particular horses and particular humans are primary substances? And is the primary substance the form of the species, which, though not a universal (1038b116), is nevertheless present in all the particular members of that species, or is it the particular form of a particular member of the species, unique to it and not present in any other member? These issues have been much debated since the 1950s, but in the 1980s and 1990s the weight of scholarly opinion shifted somewhat toward the particular-forms view, even as the widespread assumption that Metaphysics VIIVIII is a later work than the Categories came into question. (On these issues, see both Frede and Wedin; for a different view, see Loux.) The thesis that form is primary substance opens up the possibility of an inquiry, promised in book VII (1028b2733), as to whether there can be any substance entirely separate from matter. This inquiry, carried out in book XII, leads Aristotle to conclude that there are not only eternal material substances (e.g., the planets, on his view) but also eternal immaterial substances (1071b45), including Aristotle's god, the first unmoved mover whose ceaseless thinking upon thinking (1072b130) inspires the movement of the outer sphere of fixed stars (1073a2330).

Among the Stoics, by contrast, ousia in sense (2) is the single material substrate of all things, considered in abstraction from all qualities and relations depending on it (Calcidius, see Long and Hedley, Vol. 1, p. 269270; for the Stoics' debt here to Plato, Timaeus 50a5c6, see Menn, p. 216). Some Stoics also use the word ousia in sense (4), existence as opposed to nonexistence, to distinguish objects of thought that exist, objects that are peculiarly qualified portions of the material substrate ousia, for example, a particular horse, from objects of thought that are purely fictional and do not exist, for example, a centaur (Seneca, see Long and Hedley, Vol. 1, p.162).

See also Aristotle; Essence and Existence; Plato; Stoicism; Substance and Attribute.


Aristotle. The Complete Works of Aristotle, edited by Jonathan Barnes. Princeton, NJ: Princeton University Press, 1984.

Devereux, Daniel. "The Primacy of Ousia : Aristotle's Debt to Plato." In Platonic Investigations, edited by Dominic J. O'Meara. Washington, DC: Catholic University Press, 1984.

Frede, Michael. "Individuals in Aristotle." In his Essays in Ancient Philosophy. Minneapolis: University of Minnesota Press, 1987.

Long, Anthony A., and David N. Sedley. The Hellenistic Philosophers. 2 vols. Cambridge, U.K.: Cambridge University Press, 1987. This book includes both Calcidius's Plato's Timaeus: Translation And Commentary, sections 292293, p. 269270; and Seneca's Letters, 58.1315.

Loux, Michael J. Primary Ousia: An Essay on Aristotle's "Metaphysics" Z and H. Ithaca, NY: Cornell University Press, 1991.

Menn, Stephen. "The Stoic Theory of Categories." In Oxford Studies in Ancient Philosophy. Vol. 17, edited by David Sedley. Oxford, U.K.: Oxford University Press, 1999

Plato Plato: Complete Works, edited by John Cooper. Indianapolis, IN: Hackett, 1997.

Wedin, Michael. Aristotle's Theory of Substance: The "Categories" and "Metaphysics" Zeta. Oxford, U.K.: Oxford University Press, 2000.

John Driscoll (2005)