James Adam, The Republic of Plato

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πῶς γὰρ -- γνωσθείη; ‘for how can something which is not, be known?’ Cf. Parm. 132 B, C ἓν ἕκαστόν ἐστι τῶν νοημάτων, ϝόημα δὲ οὐδενός; Ἀλλ᾽ ἀδύνατον, εἰπεῖν. Ἀλλὰ τινός; Ναί. Ὄντος ἢ οὐκ ὄντος; Ὄντος. Οὐχ ἑνός τινος, ὃ ἐπὶ πᾶσιν τὸ νόημα ἐπὸν νοεῖ, μίαν τινὰ οὖσαν ἰδέαν; Ναί. “Quod Parmenides simpliciter dicit ταὐτὸν δ᾽ ἐστὶ νοεῖν τε καὶ οὕνεκέν ἐστι νόημα, id sibi prorsus probari Plato plus semel significat” Bonitz Disput. Plato. duae p. 11. That everything which is known exists in a certain sense, is of course a truism. But when Plato says that the objects of knowledge ‘are,’ the kind of οὐσία which he means is substantial, selfexistent οὐσία. If it is really Antisthenes for whom Glauco is answering (ὑπὲρ ἐκείνου ἀποκρίνου), the words πῶς—γνωσθείη are exceedingly well chosen; for Antisthenes (perhaps in his περὶ δόξης ἢ ἐπιστήμης, perhaps in Σάθων, ἢ οὐκ ἔστιν ἀντιλέγειν) had argued in almost exactly the same way to prove the impossibility of contradiction. See Proclus in Crat. 37 (Zeller^{4} II 1 p. 302 note 1) Ἀντισθένης ἔλεγεν μὴ δεῖν ἀντιλέγειν: πᾶς γάρ, φησί, λόγος ἀληθεύει: ὁ γὰρ λέγων τὶ λέγει: ὁ δὲ τὶ λέγων τὸ ὂν λέγει: ὁ δὲ τὸ ὂν λέγων ἀληθεύει and cf. Plat. Crat. 429 D. It is by no means improbable that Plato has this or some similar argument of Antisthenes in view, and feathers his arrows from his victim's wing. Antisthenes and his friends would not of course admit the connotation which Plato gives to ὄν, but Plato is not attempting to prove the Ideal theory. The object of the whole investigation is to shew that his opponents possess only δόξα, on the assumption that the theory of Ideas is true: cf. 476 A note