Note 21 — Chapter XXVIII · THEÆTETUS.
Reading edition
See this point handled in Sextus Empiric. adv. Mathemat. viii. 355-362. We may here cite a remark of Simplikius in his Commentary on the Categories of Aristotle (p. 64, a. in Schol. Brandis). Aristotle (De Animâ, iii. 2, 426, a. 19; Categor. p. 7, b. 23) lays down the doctrine that in most cases Relata or (τὰ πρός τι) are “simul Naturâ, καὶ συναναιρεῖ ἄλληλα”: but that in some Relata this is not true: for example, τὸ ἐπιστητὸν is relative to ἐπιστήμη, yet still it would seem prior to ἐπιστήμη (πρότερον ἂν δόξειε τῆς ἐπιστήμης εἶναι). There cannot be ἐπιστήμη without some ἐπιστητόν: but there may be ἐπιστητὸν without any ἐπιστήμη. There are few things, if any (he says), in which the ἐπιστητὸν (cognoscibile) is simul naturâ with ἐπιστήμη (or cognitio) and cannot be without it.
Upon which Simplikius remarks, What are these few things? Τίνα δὲ τὰ ὀλίγα ἐστίν, ἐφ’ ὧν ἅμα τῷ ἐπιστητῷ ἡ ἐπιστήμη ἐστίν; Τὰ ἄνευ ὕλης, τὰ νοητά, ἅμα τῷ κατ’ ἐνεργείαν ἀεὶ ἐστώσῃ ἐπιστήμη ἔστιν, εἴτε καὶ ἐν ἡμῖν ἐστί τις τοιαύτη ἀεὶ ἄνω μένουσα, … εἴτε καὶ ἐν τῷ κατ’ ἐνεργείαν vῷ εἴ τις καὶ τὴν νόησιν ἐκείνην ἐπιστήμην ἕλοιτο καλεῖν. δύναται δὲ καὶ διὰ τὴν τῶν κοινῶν ὑπόστασιν εἰρῆσθαι, τὴν ἐξ ἀφαιρέσεως· ἅμα γὰρ τῇ ὑποστάσει τούτων καὶ ἡ ἐπιστήμη ἐστίν. ἀληθὲς δὲ καὶ ἐπὶ τῶν ἀναπλασμάτων τῶν τε ἐν τῇ φαντασίᾳ καὶ τῶν τεχνιτῶν· ἅμα γὰρ χίμαιρα καὶ ἡ ἐπιστήμη χιμαίρας.
We see from hence that Simplikius recognises Concepts, Abstractions, and Fictions, to be dependent on the Conceiving, Abstracting, Imagining, Mind — as distinguished from objects of Sense, which he does not recognise as dependent in the like manner. He agrees in the doctrine of Protagoras as to the former, but not as to the latter. This illustrates what I have affirmed, That the Protagorean doctrine of “Homo Mensura” is not only unconnected with the other principle (that Knowledge is resolvable into sensible perception) to which Aristotle and Plato would trace it — but that there is rather a repugnance between the two. The difficulty of proving the doctrine, and the reluctance to admit it, is greatest in the case of material objects, least in the case of Abstractions, and General Ideas. Yet Aristotle, in reasoning against the Protagorean doctrine (Metaphysic. Γ. pp. 1009-1010, &c.) treats it like Plato, as a sort of corollary from the theory that Cognition is Sensible Perception.
Simplikius farther observes (p. 65, b. 14) that Aristotle is not accurate in making ἐπιστητὸν correlate with ἐπιστήμη: that in Relata, the potential correlates with the potential, and the actual with the actual. The Cognoscible is correlative, not with actual cognition (ἐπιστήμη) but with potential Cognition, or with a potential Cognoscens. Aristotle therefore is right in saying that there may be ἐπιστητὸν without ἐπιστήμη, but this does not prove what he wishes to establish.
Themistius, in another passage of the Aristotelian Scholia, reasoning against Boethus, observes to the same effect as Simplikius, that in relatives, the actual correlates with the actual, and the potential with the potential:—
Καίτοι, φησί γε ὁ Βοηθός, οὐδὲν κωλύει τὸν ἀριθμὸν εἶναι καὶ δίχα τοῦ ἀριθμοῦντος, ὥσπερ οἶμαι τὸ αἰσθητὸν καὶ δίχα τοῦ αἰσθανομένου· σφάλλεται δέ, ἅμα γὰρ τὰ πρὸς τί, καὶ τὰ δυνάμει πρὸς τὰ δυνάμει· ὥστε εἰ μὴ καὶ ἀριθμητικόν, οὐδὲ τὸ ἀριθμητόν (Schol. ad Aristot. Physic. iv. p. 223, a. p. 393, Schol. Brandis).
Compare Aristotel. Metaphysic. M. 1087, a. 15, about τὸ ἐπίστασθαι δυνάμει and τὸ ἐπίστασθαι ἐνεργείᾳ.
About the essential co-existence of relatives — Sublato uno, tollitur alterum — see also Sextus Empiric. adv. Mathematicos, vii. 395, p. 449, Fabric.