[22] Plato, Epist. vii. 341, B, C. τί τούτου κάλλιον ἐπέπρακτ’ ἂν ἡμῖν ἐν τῷ βίῳ ἢ τοῖς τε ἀνθρώποισι μέγα ὄφελος γράψαι καὶ τὴν φύσιν εἰς φῶς πᾶσι προαγαγεῖν;
[23] Plat. Epist. vii. 341 E.
[24] Plato, Epist. vii. 341 C. οὔκουν ἐμόν γε περὶ αὐτῶν ἔστι σύγγραμμα οὐδε μή ποτε γένηται· ῥητὸν γὰρ οὐδαμῶς ἐστιν ὡς ἄλλα μαθήματα, ἀλλ’ ἐκ πολλῆς συνουσίας γιγνομένης περὶ τὸ πρᾶγμα αὐτὸ καὶ τοῦ συζῇν, ἐξαίφνης, οἷον ἀπὸ πυρὸς πηδήσαντος ἐξαφθὲν φῶς, ἐν τῇ ψυχῇ γενόμενον αὐτὸ ἑαυτὸ ἤδη τρέφει.
This sentence, as a remarkable one, I have translated literally in the text: that which precedes is given only in substance.
We see in the Republic that Sokrates, when questioned by Glaukon, and urged emphatically to give some solution respecting ἡ τοῦ ἀγαθοῦ ἰδέα and ἡ τοῦ διαλέγεσθαι δύναμις, answers only by an evasion or a metaphor (Republic, vi. 506 E, vii. 533 A). Now these are much the same points as what are signified in the letter to Dionysius, under the terms τὰ πρῶτα καὶ ἄκρα τῆς φύσεως — ἡ τοῦ πρώτου φύσις (312 E): as to which Plato, when questioned, replies in a mystic and unintelligible way.
He illustrates his doctrine by the successive stages of geometrical teaching. Difficulty to avoid the creeping in of error at each of these stages.
Plato then proceeds to give an example from geometry, illustrating the uselessness both of writing and of direct exposition. In acquiring a knowledge of the circle, he distinguishes five successive stages. 1. The Name. 2. The Definition, a proposition composed of nouns and verbs. 3. The Diagram. 4. Knowledge, Intelligence, True Opinion, Νοῦς. 5. The Noumenon — Αὐτὸ-Κύκλος — ideal or intelligible circle, the only true object of knowledge.[25] The fourth stage is a purely mental result, not capable of being exposed either in words or figure: it presupposes the three first, but is something distinct from them; and it is the only mental condition immediately cognate and similar to the fifth stage, or the self-existent idea.[26]
[25] Plato, Epist. vii. 342 A, B. The geometrical illustration which follows is intended merely as an illustration, of general principles which Plato asserts to be true about all other enquiries, physical or ethical.
[26] Plat. Epist. vii. 342 C. ὡς δὲ ἓν τοῦτο αὖ πᾶν θετέον, οὐκ ἐν φωναῖς οὐδ’ ἐν σωμάτων σχήμασιν ἀλλ’ ἐν ψυχαῖς ἐνόν, ᾧ δῆλον ἕτερον τε ὂν αὐτοῦ τοῦ κύκλου τῆς φύσεως, τῶν τε ἔμπροσθεν λεχθέντων τριῶν. τούτων δὲ ἐγγύτατα μὲν ξυγγενείᾳ καὶ ὁμοιότητι, τοῦ πέμπτου (i. e. τοῦ Αὐτὸ-κύκλου) νοῦς (the fourth stage) πεπλησίακε, τἄλλα δὲ πλέον ἀπέχει.
In Plato’s reckoning, ὁ νοῦς is counted as the fourth, in the ascending scale, from which we ascend to the fifth, τὸ νοούμενον, or νοητόν. Ὁ νοῦς and τὸ νοητὸν are cognate or homogeneous — according to a principle often insisted on in ancient metaphysics — like must be known by like. (Aristot. De Animâ, i. 2, 404, b. 15.)