1. Parmenides shows, and Sokrates does not deny, that these Forms or Ideas described as absolute, self-existent, unchangeable, must of necessity be unknown and unknowable to us.[29] Whatever we do know, or can know, is relative to us; — to our actual cognition, or to our cognitive power. If you declare an object to be absolute, you declare it to be neither known nor knowable by us: if it be announced as known or knowable by us, it is thereby implied at the same time not to be absolute. If these Forms or Objects called absolute are known, they can be known only by an absolute Subject, or the Form of a cognizant Subject: that is, by God or the Gods. Even thus, to call them absolute is a misnomer: they are relative to the Subject, and the Subject is relative to them.
[29] Plato, Parmenid. 133 B. εἴ τις φαίη μηδὲ προσήκειν αὐτὰ γιγνώσκεσθαι ὄντα τοιαῦτα οἷά φαμεν δεῖν εἶναι τὰ εἴδη.… ἀπίθανος ἂν εἴη ὁ ἄγνωστα αὐτὰ ἀναγκάζων εἶναι. 134 A. ἡ δὲ παρ’ ἡμῖν ἐπιστήμη οὐ τῆς παρ’ ἡμῖν ἂν ἀληθείας εἴη; καὶ αὖ ἑκάστη ἡ παρ’ ἡμῖν ἑπιστήμη τῶν παρ’ ἡμῖν ὄντων ἑκάστου ἂν ἐπιστήμη ξύμβαινοι εἶναι; 134 C. ἄγνωστον ἄρα ἡμῖν ἔστι καὶ αὐτὸ τὸ καλὸν ὃ ἔστι, καὶ τὸ ἀγαθόν, καὶ πάντα ἃ δὴ ὡς ἰδέας αὐτὰς οὔσας ὑπολαμβάνομεν.
The opinion here advanced by the Platonic Parmenides asserts, in other words, what is equivalent to the memorable dictum of Protagoras — “Man is the measure of all things — of things existent, that they do exist — and of things non-existent, that they do not exist”. This dictum affirms universal relativity, and nothing else: though Plato, as we shall see in the elaborate argument against it delivered by Sokrates in the Theætêtus, mixed it up with another doctrine altogether distinct and independent — the doctrine that knowledge is sensible perception.[30] Parmenides here argues that if these Forms or Ideas are known by us, they can be known only as relative to us: and that if they be not relative to us, they cannot be known by us at all. Such relativity belongs as much to the world of Conception, as to the world of Perception. And it is remarkable that Plato admits this essential relativity not merely here, but also in the Sophistês: in which latter dialogue he denies the Forms or Ideas to be absolute existences, on the special ground that they are known:— and on the farther ground that what is known must act upon the knowing mind, and must be acted upon thereby, i.e., must be relative. He there defines the existent to be, that which has power to act upon something else, or to be acted upon by something else. Such relativeness he declares to constitute existence:[31] defining existence to mean potentiality.
[30] I shall discuss this in the coming [chapter] upon the Theætêtus.
[31] Plato, Sophistês, pp. 248-249. This reasoning is put into the mouth of the Eleatic Stranger, the principal person in that dialogue.
Answer of Sokrates — That Ideas are mere conceptions of the mind. Objection of Parmenides correct, though undeveloped.
2. The second point which deserves notice in this portion of the Parmenidês, is the answer of Sokrates (when embarrassed by some of the questions of the Eleatic veteran) — “That these Forms or Ideas are conceptions of the mind, and have no existence out of the mind”. This answer gives us the purely Subjective, or negation of Object: instead of the purely Objective (Absolute), or negation of Subject.[32] Here we have what Porphyry calls the deepest question of philosophy[33] explicitly raised: and, as far as we know, for the first time. Are the Forms or Ideas mere conceptions of the mind and nothing more? Or are they external, separate, self-existent realities? The opinion which Sokrates had first given declared the latter: that which he now gives declares the former. He passes from the pure Objective (i.e., without Subject) to the pure Subjective (i.e., without Object). Parmenides, in his reply, points out that there cannot be a conception of nothing: that if there be Conceptio, there must be Conceptum aliquid:[34] and that this Conceptum or Concept is what is common to a great many distinct similar Percepta.
[32] Plato, Parmenid. p. 132 A-B.
The doctrine, that ποιότητες were φιλαὶ ἔννοιαι, having no existence without the mind, was held by Antisthenes as well as by the Eretrian sect of philosophers, contemporary with Plato and shortly after him. Simplikius, Schol. ad Aristot. Categ. p. 68, a. 30, Brandis. See, respecting Antisthenes, the first volume of the present work, [p. 165].
[33] See the beginning of Porphyry’s Introduction to the Categories of Aristotle. βαθυτάτη οὔσης τῆς τοιαύτης πραγματείας, &c. Simplikius (in Schol. ad Aristot. Categ. p. 68, a. 28, ed. Brandis) alludes to the Eretrian philosophers and Theopompus, who considered τὰς ποιότητας as φιλὰς μόνας ἐννοίας διακενῶς λεγομένας κατ’ οὐδεμίας ὑποστάσεως, οἷον ἀνθρωπότητα ἢ ἱππότητα, &c.