EON. - From the Greek αἰών (for the form: Latin aevium) it indicates time, the duration of something, so that according to the varying conception of being there are various notions of e., opposed and discordant.
In the Presocratics αἰών indicates a certain "period of time," e.g., the duration of a man's life: thus Empedocles speaks of an "unceasing" e. (ἐμπεδος αἰών: fr. B 17, 8); Anaximenes (fr. A 6), Anaximander (fr. A 10) and Empedocles himself (fr. B 16, 2) speak of an "immense" e. (ἀσπετὸς αἰών). In Plato, it is not known by what process of development, αἰών indicates the eternity of which the "visible world becomes the visible image" (ἐκλῶν) and time is its intermediary (Tim. 37 d-38 b). This development is instead more visible in Aristotle: treating of the duration of celestial bodies, after having said that his predecessors with divine sense (θεῖος) had introduced αἰών to indicate the total duration proper to each being: τὸ γὰρ τέλος τὸ περὶ ἑνὸν τὸν τέλος ἑνόντων ζωῆς γρόνων, οὐ μὴ δὲν εἶκα κατὰ γόπιν...; he continues by affirming that "for the same reason" one can call αἰών the (infinite) duration of time of the whole heaven and the totality of time unto infinity: "a meaning this, Aristotle observes, fundamental and original because αἰών comes from 'being always' (ἀπὸ τοῦ ἀσὶ εἶναι) and therefore being 'immortal and eternal' (ἐβέλαστος καὶ θεῖος: De coelo, I, 9, 27ga 25-28). There is therefore still, as in Plato, the intimate connection between time and eternity, but eternity is placed within nature itself.
This was well noted by Plotinus (Enn., III, 7; περὶ αἰώνῳ καὶ γρόνου) who, taking up again the position of the Platonic Timaeus, frees αἰών from every dependence and involvement in the physical world in order to place it in direct connection with the soul and the intelligible world and to make of it a property (ἐβέλαστος); αἰών can be nothing but the quality of the absolutely immutable being of that "of which one cannot say that it has been or that it will be, but which simply is, the stable being that admits no modifications in the future and that has not changed in the past... perennial life" (Enn., loc. cit., III, 7, 3). But in the development of Neoplatonism Proclus criticizes Plotinus himself insofar as, faithful to his theory of hypotheses and to the technique of participation, he places αἰών above the intelligible and time above the soul: thus before all eternal things there exists eternity in itself and before temporal things there exists time (Procli elementatio Theologiae, propp. 52-53, ed. E. R. Dodds, Oxford 1935, p. 50 ff.). According to Dodds (loc. cit., p. 228) Proclus gathers up the whole development of Greek thought, including Aristotle, down to the magical doctrines: thus one finds the plural αἰώνες and there is a hypostatized and deified Αἰών which is its principle and almost its substance. In St. Augustine the Plotinian conception is filtered through his experience of conversion and becomes the eternity of transcendence as the point of convergence of the history both of peoples and of individual persons (infantia, pueritia, adolescentia...). In Aristotelian scholasticism, on the other hand, e. becomes the intermediate duration between time and eternity, proper to incorruptible beings (celestial bodies, intelligences) and St. Thomas calls it "aeternitas participata" (cf. Quod., V, 4, 7; Sum. Theol., 1ª, q. 10, p. 5) which therefore, with the fall of the incorruptibility of the stars, remains the exclusive property of spiritual creatures. E. is therefore the duration and the measure of the duration of finite spiritual substances which have being distinct from essence (In I Sent., dist. 19, q. 2, a. 2; ed. P. Mandonnet, I, Paris 1929, p. 471), or rather the fullness of being of their immutable essence as spiritual reality.