The purpose of this paper is to highlight the basic meaning of ousia in Plato’s philosophical use of the term. “Basic” is not intended as “the strongest”, let alone “exclusive”, insofar as the semantics of ousia encompasses a variety of philosophical meanings. On the contrary, the basic meaning is proposed to be the elementary semantic component of ousia, which is present in the background of Plato’s quasi‑technical use of the term and marks the difference from its ordinary meaning. In view of this, a “genetic” aspect is firstly considered, that is, the connection with the Socratic question “What is X ?”. Thereafter, a brief mention to Plato’s conception of language and its relation to reality is made, focusing on the notion of eponymy. This because Plato’s coinage of the philosophical semantics of ousia gives this term the role of an eponym, in the sense that its prominent value is the reference to the Forms, but this does not exclude (on the contrary, includes) that it may refer to other types of entities. In the last two sections of the paper, a number of passages from Plato’s works are examined to show that the basic meaning of ousia ultimately is “determined existence”, inasmuch as it designates any thing existing with its own peculiar feature(s). For ousiai can be empirical things as well as Forms, which implies that the meaning of Plato’s ousia should not be understood in the light of an (Aristotelian) substance/accident relation.

The Theatetus and the Sophist present in succession two “battles” regarding ousia. In so doing, ousia is placed at the heart of what is essential to both dialogues : in fact, ousia interconnects with the conditions of possibility, both physical and metaphysical, of logos and epistèmè.However, each dialogue brings differing conceptions of discourse and science into play, and both articulate a different train of thought regarding being. Ousia appears differently in the two dialogues and it is not the same thing as the notion of ousia, usually considered to be truly Platonic, presented in the central books of the Republic, which neither the Socrates of the Theatetus nor the Stranger of the Sophist put forward.Both present ways out of the battles, each has its own middle course. Against the thesis of the non‑immutability of ousia, the Theatetus establishes that there is not only motion. And, unlike the doctrine that reduces ousia to Forms excluding all motion, the Sophist shows that while there is not only motion, there is not only rest either. Such different orientations in the treatment of ousia, just below and just above doctrinal Platonism as it were, adjust to distinctions in the epistemological stakes : even if the Theatetus emphasizes that science proceeds from an activity of the soul bringing together “commons”, it is not yet a question, as it will be the following day, of systematically getting epistèmè to intellectually grasp a set of eidetic relationships, nor of making logos itself the elaboration of relationships. One needs to have left the field where those in favour of motion are challenged on their own ground, and in an albeit transformed field marked out by the partisans of eide which is not the field hierarchical metaphysics either, it will become possible to analyse the discourse itself as a relational framework that is consistent with the framework of ousia.

This article examines the problem of the status of the different degrees of being that Plato, in the Timaeus, seems to attribute to the different kinds of reality that he distinguishes. In what sense and under what conditions is it possible to state that the intelligible forms, the sensible things and the spatial and material substratum of the chora “are” and “exist” ?

Which Greek concept is translated by essentia ? The question is raised from a methodological perspective and aims at re‑examining the Latin texts on which philologists have based the history of essentia. Neither Cicero nor Seneca used the term, because they did not need it: its philosophical meaning is fully developed only when the theological discussions about the Trinity arise. The absence of essentia in the classical period gives some useful information about the way Plato was read at Rome: thus a critical history of the Latin philosophical lexicon should contribute to a better understanding of the reception of Greek philosophy in Rome.

Expressing ‘essence’ in the translation and commentary of Timaeus by Calcidius (4th c. p.D.) depends on Platonist terminology which is not completely stabilised. We will see how, in his translation, Calcidius translated Greek words as οὐσία or φύσις, but also how he used the word substantia whereas there was no expression of essence in the Greek text. The Latin commentator used both essentia and substantia, but the latter quite often. However, in doing so, he weakened the meaning of the word substantia. Lastly, Calcidius translated into Latin and used Greek no‑Platonist expressions with a very specific signification of ‘essence’.

In Aristotle, substance, being specified in Z17 as cause and principle, is to be understood according to the analogical theory of principles and causes, namely form, matter and privation. These three causes involve potentiality and actuality, since form, privation, and the compound substance are in actuality, while matter is in potentiality. ≪What a substance is≫ depends on the connection between these three principles. In order to grasp the meaning of this connection, one has to put the analogical theory of principles back in its context, where previous theories on contraries (Plato’s theory included) are amended.The amendment of previous theories of principles relies on positing a third term, matter, between both opposites, i.e. form and privation. The implied distinction between matter and privation allows an understanding of generation which makes it compatible with substance. While generation removes privation, substance as form gives shape to matter, final matter and shape being identical to one another. Predication of matter by form supplies a relevant pattern for considering the relationship between matter, form and privation. At the same time, predication of matter by form provides both a renovated theory of opposites and a new theory of form as a cause, i.e. a theory of form as actuality.