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.
Evidence from Plato proving implication of Subject and Object, in regard to the intelligible world.
That this general doctrine is true, not merely respecting the facts of sense, but also respecting the facts of mental conception, opinion, intellection, cognition — may be seen by the reasoning of Plato himself in other dialogues. How, for example, does Plato prove, in his Timæus, the objective reality of Ideas or Forms? He infers them from the subjective facts of his own mind. The subjective fact called Cognition (he argues) is generically different from the subjective fact called True Opinion: therefore the Object correlating with the One must be distinct from the Object correlating with the other: there must be a Noumenon or νοητόν τι correlating with Nous, distinct from the δοξαστόν τι which correlates with δόξα.[22] So again, in the Phædon,[23] Sokrates proves the pre-existence of the human soul from the fact that there were pre-existent cognizable Ideas: if there were knowable Objects, there must also have been a Subject Cognoscens or Cognitionis capax. The two are different aspects of one and the same conception: upon which we may doubtless reason abstractedly under one aspect or under the other, though they cannot be separated in fact. Now Both these two inferences of Plato rest on the assumed implication of Subject and Object.[24]
[22] Plato, Timæus, p. 51 B-E, compare Republic, v. p. 477.