See this reasoning of Plato set forth in Zeller, Die Phil. der Griech. vol. ii. pp. 412-416, ed. 2nd.
Nous, according to Plato (Tim. 51 E), belongs only to the Gods and to a select few among mankind. It is therefore only to the Gods and to these few men that Νοητὰ exist. To the rest of mankind Νοητὰ are non-apparent and non-existent.
[23] Plato, Phædon, pp. 76-77. ἴση ἀνάγκη ταῦτά τε (Ideas or Forms) εἶναι, καὶ τὰς ἡμετέρας ψυχὰς πρὶν καὶ ἡμᾶς γεγονέναι — καὶ εἰ μὴ ταῦτα, οὐδὲ τάδε. Ὑπερφυῶς, ἔφη ὁ Σιμμίας, δοκεῖ μοι ἡ αὐτὴ ἀνάγκη εἶναι, καὶ εἰς καλόν γε καταφεύγει ὁ λόγος εἰς τὸ ὁμοίως εἶναι τήν τε ψυχὴν ἡμῶν πρὶν γενέσθαι ἡμᾶς καὶ τὴν οὐσίαν ἣν σὺ νῦν λέγεις.
Compare p. 92 E of the same dialogue with the notes of Wyttenbach and Heindorf — “Haec autem οὐσία Idearum, rerum intelligibilium, αὐτῆς ἐστὶν (sc. τῆς ψυχῆς) ut hoc loco dicitur, est propria et possessio animæ nostræ,” &c.
About the essential implication of Νοῦς with the Νοητά, as well as of τὸ δόξαζον with τὰ δοξαζόμενα, and of τὸ αἰσθανόμενον with τὰ αἰσθητά, see Plutarch, De Animæ Procreat. in Timæo, pp. 1012-1024; and a curious passage from Joannes Philoponus ad Aristot. Physica, cited by Karsten in his Commentatio De Empedoclis Philosophiâ, p. 372, and Olympiodorus ad Platon. Phædon, p. 21. τὸν νοῦν φαμὲν ἀκριβῶς γινώσκειν, διότι αὐτός ἐστι τὸ νοητόν.
Sydenham observes, in a note upon his translation of the Philêbus (note 76, p. 118), “Being Intelligent and Being Intelligible are not only correlatives, but are so in their very essence: neither of them can be at all, without the Being of the other”.
[24] I think that the inference in the Phædon is not necessary to prove that conclusion, nor in itself just. For when I speak of Augustus and Antony as having once lived, and as having fought the battle of Actium, it is noway necessary that I should believe myself to have been then alive and to have seen them: nor when I speak of civil war as being now carried on in the United States of America, is it necessary that I should believe myself to be or to have been on the spot as a percipient witness. I believe, on evidence which appears to me satisfactory, that both these are real facts: that is, if I had been at Actium on the day of the battle, or if I were now in the United States, I should see and witness the facts here affirmed. These latter words describe the subjective side of the fact, without introducing any supposition that I have been myself present and percipient.
The Protagorean measure is even more easily shown in reference to the intelligible world than in reference to sense.
In truth, the Protagorean measure or limit is even more plainly applicable to our mental intuitions and mental processes (remembering, imagining, conceiving, comparing, abstracting, combining of hypotheses, transcendental or inductive) than to the matter of our sensible experience.[25] In regard to the Entia Rationis, divergence between one theorist and another is quite as remarkable as the divergence between one percipient and another in the most disputable region of Entia Perceptionis. Upon the separate facts of sense, there is a nearer approach to unanimity among mankind, than upon the theories whereby theorising men connect together those facts to their own satisfaction. An opponent of Protagoras would draw his most plausible arguments from the undisputed facts of sense. He would appeal to matter and what are called its primary qualities, as refuting the doctrine. For in describing mental intuitions, Mind or Subject cannot well be overlaid or ignored: but in regard to the external world, or material substance with its primary qualities, the objective side is so lighted up and magnified in the ordinary conception and language — and the subjective side so darkened and put out of sight — that Object appears as if it stood single, apart, and independent.
[25] Bacon remarks that the processes called mental or intellectual are quite as much relative to man as those called sensational or perceptive. “Idola Tribûs sunt fundata in ipsâ naturâ humanâ. Falso enim asseritur, Sensum humanum esse mensuram rerum: quin contra, omnes perceptiones, tam Sensûs quam Mentis, sunt ex analogiâ hominis, non ex analogiâ Universi.”