[143] Plato, Sophist. p. 249 C-D. Τῷ δὴ φιλοσόφῳ καὶ ταῦτα μάλιστα τιμῶντι πᾶσα ἀνάγκη διὰ ταῦτα μήτε τῶν ἓν ἢ καὶ τὰ πολλὰ εἴδη λεγόντων τὸ πᾶν ἑστηκὸς ἀποδέχεσθαι, τῶν τε αὖ πανταχῇ τὸ ὂν κινοῦντων μηδὲ τὸ παράπαν ἀκούειν· ἀλλὰ κατὰ τὴν τῶν παίδων εὐχήν, ὅσα ἀκίνητά τε καὶ κεκινημένα, τὸ ὄν τε καὶ τὸ πᾶν, ξυναμφότερα λέγειν.
Ritter states the result of this portion of the Sophistês correctly. “Es bleibt uns als Ergebniss aller dieser Untersuchungen über das Seyn, dass die Wahrheit sowohl des Werdens, als auch des beharrlichen Seyns, anerkannt werden müsse” (Geschichte der Philos. ii. p. 281).
The Sophistês recedes from the Platonic point of view, and approaches the Aristotelian.
That the Eleate in the Sophistes recedes from the Platonic point of view and approaches towards the Aristotelian, will be seen also if we look at the lesson of logic which he gives to Theætêtus. In his analysis of a proposition — and in discriminating such conjunctions of words as are significant, from such as are insignificant — he places himself on the same ground as that which is travelled over by Aristotle in the Categories and the treatise De Interpretatione. That the handling of the topic by Aristotle is much superior, is what we might naturally expect from the fact that he is posterior in time. But there is another difference between the two which is important to notice. Aristotle deals with this topic, as he does with every other, in the way of methodical and systematic exposition. To expound it as a whole, to distribute it into convenient portions each illustrating the others, to furnish suitable examples for the general principles laid down — are announced as his distinct purposes. Now Plato’s manner is quite different. Systematic exposition is not his primary purpose: he employs it up to a certain point, but as means towards another and an independent purpose — towards the solution of a particular difficulty, which has presented itself in the course of the dialogue. — “Nosti morem dialogorum.” Aristotle is demonstrative: Plato is dialectical. In our present dialogue (the Sophistês), the Eleate has been giving a long explanation of Non-Ens; an explanation intended to prove that Non-Ens was a particular sort of Ens, and that there was therefore no absurdity (though Parmenides had said that this was absurdity) in assuming it as a passable object of Cognition, Opination, Affirmation. He now goes a step further, and seeks to show that it is, actually and in fact, an object of Opination and Affirmation.[144] It is for this purpose, and for this purpose only, that he analyses a proposition, specifies the constituent elements requisite to form it, and distinguishes one proposition from another.
[144] Plato, Sophist. p. 261 D.
Accordingly, the Eleate, — after pointing out that neither a string of nouns repeated one after the other, nor a string of verbs so repeated, would form a significant proposition, — declares that the conjunction of a noun with a verb is required to form one; and that opination is nothing but that internal mental process which the words of the proposition express. The smallest proposition must combine a noun with a verb:— the former signifying the agent, the latter, the action or thing done.[145] Moreover, the proposition must be a proposition of something; and it must be of a certain quality. By a proposition of something, Plato means, that what is called technically the subject of the proposition (in his time there were no technical terms of logic) must be something positive, and cannot be negative: by the quality of the proposition, he means that it must be either true or false.[146]
[145] Plato, Sophist. p. 262 C.
[146] Plato, Sophist. p. 262 E. Λόγον ἀναγκαῖον, ὅταν περ ᾖ, τινὸς εἶναι λόγον, μὴ δέ τινος, ἀδύνατον … Οὐκοῦν καὶ ποιόν τινα αὐτὸν εἶναι δεῖ; Compare p. 237 E.
In the words here cited Plato unconsciously slides back into the ordinary acceptation of μή τι: that is, to μὴ in the sense of negation. If we adopt that peculiar sense of μή, which the Eleate has taken so much pains to prove just before in the case of τὸ μὴ ὂν (that is, if we take μὴ as signifying not negation but simply difference), the above argument will not hold. If τίς signifies one subject (A), and μή τις signifies simply another subject (B) different from A (ἕτερον), the predicate ἀδύνατον cannot be affirmed. But if we take μή τις in its proper sense of negation, the ἀδύνατον will be so far true that οὐκ ἄνθρωπος, οὐ Θεαίτητος, cannot be the subject of a proposition. Aristotle says the same in the beginning of the Treatise De Interpretatione (p. 16, a. 30).
Aristotle assumes without proof, that there are some propositions true, others false.