IDENTITY. – It indicates the reflected unity of being in the variety of its forms and in the multiplicity of its elements, and may be either logical or real (Aristotle, Met., V, 9, 1018 a 7). It is therefore the expression of the persistence, or constancy, of something with itself. Identity may be considered at all degrees and in all forms of being, and from the various points of view that it presents, so that identity in one order may well entail multiplicity and diversity in another order: one single and identical substance may in fact possess, both simultaneously and successively, a multiplicity of different accidents, and yet for this reason it cannot lose its identity. The problem of identity therefore coincides with that of being (the problem of the one and the many), and it is in this particular form of reconciling the one and the multiple that it especially occupied Greek speculation.
Among the pre-Socratics, identity appears as a “commonality” of a physical order, as a certain determinate reality that forms the “substratum” (ὀποκείμενον) of the external variety of phenomena. When Thales asserts that “water is the principle of beings” (ἀρχὴν τῶν ὄντων ἀπεφήνετο τὸ ὕδωρ; Doc. graeci, ed. H. Diels, Berlino 1929, 276 a 5), he intends to refer to water everything real that seems to belong to entities, both with regard to being and with regard to becoming. This, so to speak, “dynamic” resolution of identity is common to all the Naturales, probably with the exception of Parmenides, for whom reality was posited in the form of immobility, so that becoming was relegated to the world of unreal appearance: being announces itself only according to “one single way, that which is, is,” and its properties are: “unbegotten,” “indestructible,” entirely gathered into its “structure” and “unshaken” (Fragm. der Vorsokratiker, ed. H. Diels-W. Kranz, I, 2ª ed., Berlino 1934, p. 235, 2 sgg.). The metaphysical meaning of identity is therefore the following: everything in the world is contrariety and multiplicity; but contraries exclude one another; hence this world of contraries is false, and only the eternally unchanging ὄν is true. Heraclitus, by contrast, thinks that contraries condition one another: this is the great mystery of being, namely, that opposition is unity, and that everything harmonizes in the process of its oppositions. Opposition—not identity—is therefore the essence of all things, and the world of oppositions is the only true and real world: “from all things comes the one, and from the one all things” (καὶ ἐκ πάντων ἐν καὶ ἐξ ἑνὸς πάντα: Fr. B, 10, op. cit., I, p. 153, 11). The Socratic systems, with Plato and Aristotle, resolve identity by proceeding from the sensible to the intelligible, in the affirmation of a higher “unity of multiplicity”; but it must be said that in this resolution, whereas Plato stands on the line of thought of Parmenides and admits no coexistence of contraries in being as such, Aristotle sees in every being of nature and in its becoming the presence and struggle of contraries, in accordance with Heraclitus’ intuition. Matter, in fact, which is the substratum of becoming, is not χωριστή like Anaximander’s ο ἄπειρον (Phys., II, 1, 329 a 8-14), but is always affected by contrariety (ἀλλ'ἀεὶ μὲν ἐνεντιώσεως [op. cit., 329 a 25-26], that is, ἀχώριστος μὲν ὑποκείμενον δὲ τοῖς ἐνεντίοις [loc. cit., 329 a 30-31]). According to the various modes of diversity, Aristotle also distinguishes various forms of identity: numerical (κατ' ἀριθμῶν), specific (τῷ εἶδει), or generic according to matter (τῇ ὕλῃ, cf. Met., X, 3, 1054 a 32). Nevertheless, despite so great an effort to ground the unity of being, Aristotle remains within dualism (matter and form, nature and spirit), since he was unable to conceive creation: later philosophies attain the identity of the entity either because they opt for matter (Stoicism, Epicureanism) or because they reduce everything to spirit (Neoplatonism). Only with the Christian doctrine of creation is there a positing of identity in duality and of duality in identity: the creature is distinct from God and at the same time His total effect.
A systematic philosophy of identity begins with Bruno’s pantheism, develops with the Spinozian theory of substance, and takes systematic form with German transcendental idealism, up to the extreme resolution of Gentile’s pure, and therefore absolutely unique, act: dialectical identity for Hegel, insofar as the “concept” (Begriff), or concrete thought, is posited as the “new immediacy” at which dialectical mediation terminates, as the in-itself-and-for-itself being of Spirit (Hegel, Wissenschaft der Logik, II, 2ª ed., ed. G. Lasson, Lipsia 1934, p. 213). Whereas for Hegel identity is a “result,” for Schelling—and for the philosophies of intuition—identity is instead a point of departure. It can therefore be understood why philosophies of an “explanatory type” (e.g., Plato, Schelling) have as their point of reference the principle of identity, whereas those of a “constructive type” (Aristotle, Hegel) prefer the principle of contradiction. In any case, it is not the principle of identity in its logical meaning (common to all philosophies) that sets the various philosophies in opposition or unites them; rather, it is the content attributed to being, whose identity and diversity within the intelligible order philosophy precisely has the task of indicating.