Aristotle   says in the Metaphysics that the old question: τί τὸ ὄν, “what is the being?” is really the question concerning the being of beings: τίς ἡ οὐσία. Aristotle brings scientific research, for the first time, to this ground, a ground that even Plato never noticed. [Heidegger  , GA18:26]

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τὸ τί ἦν εἶναι: Aristotle did not invent this term; rather, it was handed down to him by tradition. Τὸ τί ἦν εἶναι is a character of being, specifically that character on the basis of which λóγος as ὁρισμóς addresses beings. The τὸ τί ἦν εἶναι is in particular^? the topic of ὁρισμóς. An extensive understanding of this being-character is not to be expected here, but perhaps it will appear to us at the end of the lecture. I will only characterize the meaning of this being-character and its context in an entirely superficial way with the following. It refers to “being,” that is, the “what-being as it was already.” It means a being in itself, that is, with respect to what it was already, from which it stems in its being, with respect to its descent, its having come into being there. Therefore, τὸ τί ἦν εἶναι is the “being of a particular,” οὐσία ἑκάστου, which is not “everything,” or even “what is singular” or “what is individual.” With such translations, one has gone astray. [Heidegger, GA18:32]

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The τὸ τί ἦν εἶναι has in itself the determination of the ἦν: the being-there of a being, and indeed with an eye to what it was, to its descent. If the human being is determined as ζῷον λóγον ἔχον, the speaking comes in this way from its ζῷον-, being a “living thing”; this is its γένος. I see a being that is there with respect to its being, in the way that it is there as coming from out of . . . I see a being that is there genuinely in its being when I see it in its history, the being that is there in this way coming from out of its history into being. This being that is there, as there in this way, is complete; it has come to its end, to its completedness, just as the house is complete in its εἶδος as ποιούμενον. The ὑποκείμενον is already complete; I need not produce it. The body has its completedness through the surface. [Heidegger, GA18:35]

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L’expression τὸ τί est une abréviation de la locution τὸ τί ἂν εἶναι employée par Aristote dans la Métaphysique (VII, 4) pour désigner la forme essentielle. Pour rendre en français la formule grecque, on est obligé d’avoir recours au terme scolastique de quiddité. Il a été inventé pour servir d’équivalent τὸ τί ἦν εἶναι, en exprimant ce qu’une chose est selon le quid, selon l’être, et non pas selon le quale, le quantum, etc. Voy. M. Ravaisson, t. 1, p. 149-150. [Bouillet  ]

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LÉXICO: quididade^?; quid; quidam; quidditas