Plato never expected to make his Ideas fit on to the facts of sense: Aristotle tried to do it and partly succeeded.
Plato has not followed out the hint which he has here put into the mouth of Sokrates in the Parmenidês — That the Ideas or Forms are conceptions existing only in the mind. Though the opinion thus stated is not strictly correct (and is so pointed out by himself), as falling back too exclusively on the subjective — yet if followed out, it might have served to modify the too objective and absolute character which in most dialogues (though not in the Sophistês) he ascribes to his Forms or Ideas: laying stress upon them as objects — and as objects not of sensible perception — but overlooking or disallowing the fact of their being relative to the concipient mind. The bent of Plato’s philosophy was to dwell upon these Forms, and to bring them into harmonious conjunction with each other: he neither took pains, nor expected, to make them fit on to the world of sense. With Aristotle, on the contrary, this last-mentioned purpose is kept very generally in view. Amidst all the extreme abstractions which he handles, he reverts often to the comparison of them with sensible particulars: indeed Substantia Prima was by him, for the first time in the history of philosophy, brought down to designate the concrete particular object of sense: in Plato’s Phædon, Republic, &c, the only Substances are the Forms or Ideas.
Continuation of the Dialogue — Parmenides admonishes Sokrates that he has been premature in delivering a doctrine, without sufficient preliminary exercise.
Parmenides now continues the debate. He has already fastened upon Sokrates several difficult problems: he now proposes a new one, different and worse. Which way are we to turn then, if these Forms be beyond our knowledge? I do not see my way (says Sokrates) out of the perplexity. The fact is, Sokrates (replies Parmenides), you have been too forward in producing your doctrine of Ideas, without a sufficient preliminary exercise and enquiry. Your love of philosophical research is highly praiseworthy: but you must employ your youth in exercising and improving yourself, through that continued philosophical discourse which the vulgar call useless prosing: otherwise you will never attain truth.[41] You are however right in bestowing your attention, not on the objects of sense, but on those objects which we can best grasp in discussion, and which we presume to exist as Forms.[42]
[41] Plato, Parmenid. p. 135 C. Πρῲ γάρ, πρὶν γυμνασθῆναι, ὦ Σώκρατες, ὁρίζεσθαι ἐπιχειρεῖς καλόν τέ τι καὶ δίκαιον καὶ ἀγαθὸν καὶ ἓν ἕκαστον τῶν εἰδῶν … καλὴ μὲν οὖν καὶ θεία, εἶ ἴσθι, ἡ ὁρμὴ ἣν ὁρμᾷς ἐπὶ τοὺς λόγους· ἕλκυσον δὲ σαυτὸν καὶ γυμνάσαι, μᾶλλον διὰ τῆς δοκούσης ἀχρήστου εἶναι καὶ καλουμένης ὑπὸ τῶν πολλῶν ἀδολεσχίας, ἕως ἔτι νέος εἶ· εἰ δὲ μὴ, σὲ διαφεύξεται ἡ ἀλήθεια.
[42] Plato, Parmenid. p. 135 E.
What sort of exercise? Parmenides describes: To assume provisionally both the affirmative and the negative of many hypotheses about the most general terms, and to trace the consequences of each.
What sort of exercise must I go through? asks Sokrates. Zeno (replies Parmenides) has already given you a good specimen of it in his treatise, when he followed out the consequences flowing from the assumption — “That the self-existent and absolute Ens is plural”. When you are trying to find out the truth on any question, you must assume provisionally, first the affirmative and then the negative, and you must then follow out patiently the consequences deducible from one hypothesis as well as from the other. If you are enquiring about the Form of Likeness, whether it exists or does not exist, you must assume successively both one and the other;[43] marking the deductions which follow, both with reference to the thing directly assumed, and with reference to other things also. You must do the like if you are investigating other Forms — Unlikeness, Motion, and Rest, or even Existence and Non-Existence. But you must not be content with following out only one side of the hypothesis: you must examine both sides with equal care and impartiality. This is the only sort of preparatory exercise which will qualify you for completely seeing through the truth.[44]
[43] Plato, Parmenid. p. 136 A. καὶ αὖθις αὖ ἐὰν ὑποθῇ, εἰ ἔστιν ὁμοιότης ἢ εἰ μή ἐστι, τί ἐφ’ ἑκατέρας τῆς ὑποθέσεως συμβήσεται, καὶ αὐτοῖς τοῖς ὑποτεθεῖσι καὶ τοῖς ἄλλοις καὶ πρὸς αὑτὰ καὶ πρὸς ἄλληλα.
[44] Plato, Parmenid. p. 136 B.