Proklus here adopts and repeats Plato’s erroneous idea of the negative proposition and its function. When I deny that Caius is just, wise, &c., my denial does not intimate simply that I know him to be something different from just, wise; for he may have fifty different attributes, co-existent and consistent with justice and wisdom.
To employ the language of Aristotle (see a pertinent example, Physic. i. 8, 191, b. 15, where he distinguishes τὸ μὴ ὂν καθ’ αὑτὸ from τὸ μὴ ὂν κατὰ συμβεβηκός), we may say that it is not of the essence of the Different to deny or exclude that from which it is different: the Different may deny or exclude, but that is only by accident — κατὰ συμβεβηκός. Plato includes, in the essence of the Different, that which belongs to it only by accident.
Aristotle in more than one place distinguishes διαφορὰ from ἐναντίωσις — not always in the same language. In Metaphysic. I. p. 1055 a. 33, he considers that the root of all ἐναντίωσις is ἕξις and στέρησις, understood in the widest sense, i.e. affirmative and negative. See Bonitz, not. ad loc., and Waitz, ad Categor. p. 12, a. 26. The last portion of the treatise Περὶ Ἑρμηνείας was interpreted by Syrianus with a view to uphold Plato’s opinion here given in the Sophistes (Schol. ad Aristot. p. 136, a. 15 Brandis).
[129] Plato, Sophist. p. 258 B. οὐκ ἐναντίον ἐκείνῳ σημαίνουσα, ἀλλὰ τοσοῦτον μόνον, ἕτερον ἐκείνου.
If we look to the Euthydêmus we shall see that this confusion between what is different from A, and what is incompatible with or exclusive of A, is one of the fallacies which Plato puts into the mouth of the two Sophists Euthydêmus and Dionysodôrus, whom he exhibits and exposes in that dialogue. Ἄλλο τι οὖν ἕτερος, ἦ δ’ ὅς (Dionysodorus), ὢν λίθου, οὐ λίθος εἶ; καὶ ἕτερος ὢν χρυσοῦ, οὐ χρυσὸς εἶ; Ἔστι ταῦτα. Οὐκοῦν καὶ ὁ Χαιρέδημος, ἔφη, ἕτερος ὢν πατρός, οὐκ ἂν πατὴρ εἴη; (Plat. Euthydem. p. 298 A).
[130] Plato, Sophist. p. 257 B.
[131] Plato, Sophist. pp. 257 E, 258 A.
Ὄντος δὴ πρὸς ὂν ἀντίθεσις, ὡς ἔοικ’, εἶναι ξυμβαίνει τὸ μὴ καλόν.…
Ὁμοίως ἄρα τὸ μὴ μέγα, καὶ τὸ μέγα αὐτὸ εἶναι λεκτέον.
Plato distinctly recognises here Forms or Ideas τῶν ἀποφάσεων, which the Platonists professed not to do, according to Aristotle, Metaphys. A. 990, b. 13 — see the instructive Scholia of Alexander, p. 565, a. Brandis.