Now the distribution made by Thrasyllus of the dialogues under two general heads (1. Dialogues of Search or Investigation, 2. Dialogues of Exposition) coincides, to a considerable extent, with the two distinct intellectual methods recognised by Aristotle as Dialectic and Demonstrative: Dialectic being handled by Aristotle in the Topica, and Demonstration in the Posterior Analytica. “Dialectic” (says Aristotle) “is tentative, respecting those matters of which philosophy aims at cognizance.” Accordingly, Dialectic (as well as Rhetoric) embraces all matters without exception, but in a tentative and searching way, recognising arguments pro as well as con, and bringing to view the antithesis between the two, without any preliminary assumption or predetermined direction, the questioner being bound to proceed only on the answers given by the respondent: while philosophy comes afterwards, dividing this large field into appropriate compartments, laying down authoritative principia in regard to each, and deducing from them, by logical process, various positive results.[47] Plato does not use the term Dialectic exactly in the same sense as Aristotle. He implies by it two things: 1. That the process shall be colloquial, two or more minds engaged in a joint research, each of them animating and stimulating the others. 2. That the matter investigated shall be general — some general question or proposition: that the premisses shall all be general truths, and that the objects kept before the mind shall be Forms or Species, apart from particulars.[48] Here it stands in contrast with Rhetoric, which aims at the determination of some particular case or debated course of conduct, judicial or political, and which is intended to end in some immediate practical verdict or vote. Dialectic, in Plato’s sense, comprises the whole process of philosophy. His Dialogues of Search correspond to Aristotle’s Dialectic, being machinery for generating arguments and for ensuring that every argument shall be subjected to the interrogation of an opponent: his Dialogues of Exposition, wherein some definite result is enunciated and proved (sufficiently or not), correspond to what Aristotle calls Demonstration.
[47] Aristot. Metaphys. Γ. 1004, b. 25. ἔστι δὲ ἡ διαλεκτικὴ πειραστικὴ, περὶ ὧν ἡ φιλοσοφία γνωριστική. Compare also Rhet. i. 2, p. 1356, a. 33, i. 4, p. 1359, b. 12, where he treats Dialectic (as well as Rhetoric) not as methods of acquiring instruction on any definite matter, but as inventive and argumentative aptitudes — powers of providing premisses and arguments — δυνάμεις τινὲς τοῦ πορίσαι λόγους. If (he says) you try to convert Dialectic from a method of discussion into a method of cognition, you will insensibly eliminate its true nature and character:—ὅσῳ δ’ ἄν τις ἢ τὴν διαλεκτικὴν ἢ ταύτην, μὴ καθάπερ ἂν δυνάμεις ἀλλ’ ἐπιστήμας πειρᾶται κατασκευάζειν, λήσεται τὴν φύσιν αὐτῶν ἀφανίσας, τῷ μεταβαίνειν ἐπισκευάζων εἰς ἐπιστήμας ὑποκειμένων τινῶν πραγμάτων, ἀλλὰ μὴ μόνον λόγων.
The Platonic Dialogues of Search are δυνάμεις τοῦ πορίσαι λόγους. Compare the Proœmium of Cicero to his Paradoxa.
[48] Plato, Republ. vi. 511, vii. 582. Respecting the difference between Plato and Aristotle about Dialectic, see Ravaisson — Essai sur la Métaphysique d’Aristote — iii. 1, 2, p. 248.
Classification of Thrasyllus in its details. He applies his own principles erroneously.
If now we take the main scheme of distributing the Platonic Dialogues, proposed by Thrasyllus — 1. Dialogues of Exposition, with an affirmative result; 2. Dialogues of Investigation or Search, without an affirmative result — and if we compare the number of Dialogues (out of the thirty-six in all), which he specifies as belonging to each — we shall find twenty-two specified under the former head, and fourteen under the latter. Moreover, among the twenty-two are ranked Republic and Leges: each of them greatly exceeding in bulk any other composition of Plato. It would appear thus that there is a preponderance both in number and bulk on the side of the Expository. But when we analyse the lists of Thrasyllus, we see that he has unduly enlarged that side of the account, and unduly contracted the other. He has enrolled among the Expository — 1. The Apology, the Epistolæ, and the Menexenus, which ought not properly to be ranked under either head. 2. The Theætêtus, Parmenidês, Hipparchus, Erastæ, Minos, Kleitophon — every one of which ought to be transferred to the other head. 3. The Phædrus, Symposion, and Kratylus, which are admissible by indulgence, since they do indeed present affirmative exposition, but in small proportion compared to the negative criticism, the rhetorical and poetical ornament: they belong in fact to both classes, but more preponderantly to one. 4. The Republic. This he includes with perfect justice, for the eight last books of it are expository. Yet the first book exhibits to us a specimen of negative and refutative dialectic which is not surpassed by anything in Plato.
On the other hand, Thrasyllus has placed among the Dialogues of Search one which might, with equal or greater propriety, be ranked among the Expository — the Protagoras. It is true that this dialogue involves much of negation, refutation, and dramatic ornament: and that the question propounded in the beginning (Whether virtue be teachable?) is not terminated. But there are two portions of the dialogue which are, both of them, decided specimens of affirmative exposition — the speech of Protagoras in the earlier part (wherein the growth of virtue, without special teaching or professional masters, is elucidated) — and the argument of Sokrates at the close, wherein the identity of the Good and the Pleasurable is established.[49]
[49] We may remark that Thrasyllus, though he enrols the Protagoras under the class Investigative, and the sub-class Agonistic, places it alone in a still lower class which he calls Ἐνδεικτικός. Now, if we turn to the Platonic dialogue Euthydêmus, p. 278 D, we shall see that Plato uses the words ἐνδείξομαι and ὑφηγήσομαι as exact equivalents: so that ἐνδεικτικὸς would have the same meaning as ὑφηγητικός.
The classification, as it would stand, if his principles were applied correctly.
If then we rectify the lists of Thrasyllus, they will stand as follows, with the Expository Dialogues much diminished in number:—