This early example of rudimentary grammatical or logical analysis, recognising only the two main and principal parts of speech, is interesting as occurring prior to Aristotle; by whom it is repeated in a manner more enlarged, systematic,[147] and instructive. But Aristotle assumes, without proof and without supposing that any one will dispute the assumption — that there are some propositions true, other propositions false: that a name or noun, taken separately, is neither true nor false:[148] that propositions (enunciations) only can be true or false.
[147] Aristotel. De Interpr. init. with Scholia of Ammonius, p. 98, Bekk.
[148] In the Kratylus of Plato Sokrates maintains that names may be true or false as well as propositions, pp. 385 D, 431 B.
Plato in the Sophistês has undertaken an impossible task — He could not have proved, against his supposed adversary, that there are false propositions.
The proceeding of Plato in the Sophistês is different. He supposes a Sophist who maintains that no proposition either is false or can be false, and undertakes to prove against him that there are false propositions: he farther supposes this antagonist to reject the evidence of sense and visible analogies, and to acknowledge no proof except what is furnished by reason and philosophical deduction.[149] Attempting, under these restrictions, to prove his point, Plato’s Eleatic disputant rests entirely upon the peculiar meaning which he professes to have shown to attach to Non-Ens. He applies this to prove that Non-Ens may be predicated as well as Ens: assuming that such predication of Non-Ens constitutes a false proposition. But the proof fails. It serves only to show that the peculiar meaning ascribed by the Eleate to Non-Ens is inadmissible. The Eleate compares two distinct propositions — Theætêtus is sitting down — Theætêtus is flying. The first is true: the second is false. Why? Because (says the Eleate) the first predicates Ens, the second predicates Non-Ens, or (to substitute his definition of Non-Ens) another Ens different from the Ens predicated in the first.[150] But here the reason assigned, why the second proposition is false, is not the real reason. Many propositions may be assigned, which predicate attributes different from the first, but which are nevertheless quite as much true as the first. I have already observed, that the reason why the second proposition is false is, because it contradicts the direct testimony of sense, if the persons debating are spectators: if they are not spectators, then because it contradicts the sum total of their previous sensible experience, remembered, compared, and generalised, which has established in them the conviction that no man does or can fly. If you discard the testimony of sense as unworthy of credit (which Plato assumes the Sophist to do), you cannot prove that the second proposition is false — nor indeed that the first proposition is true. Plato has therefore failed in giving that dialectic proof which he promised. The Eleate is forced to rely (without formally confessing it), on the testimony of sense, which he had forbidden Theætêtus to invoke, twenty pages before.[151] The long intervening piece of dialectic about Ens and Non-Ens is inconclusive for his purpose, and might have been omitted. The proposition — Theætêtus is flying — does undoubtedly predicate attributes which are not as if they were,[152] and is thus false. But then we must consult and trust the evidence of our perception: we must farther accept are not in the ordinary sense of the words, and not in the sense given to them by the Eleate in the Platonic Sophistês. His attempt to banish the specific meaning of the negative particle, and to treat it as signifying nothing more than difference, appears to me fallacious.[153]
[149] Plato, Sophist. p. 240 A. It deserves note that here Plato presents to us the Sophist as rejecting the evidence of sense: in the Theætêtus he presents to us the Sophist as holding the doctrine ἐπιστήμη = αἴσθησις. How these propositions can both be true respecting the Sophists as a class I do not understand. The first may be true respecting some of them; the second may be true respecting others; respecting a third class of them, neither may be true. About the Sophists in a body there is hardly a single proposition which can be safely affirmed.
[150] Plato, Sophist. p. 263 C.
[151] Theætêtus makes this attempt and is checked by the Eleate, pp. 239-240. It is in p. 261 A that the Eleate begins his proof in refutation of the supposed Sophist — that δόξα and λόγος may be false. The long interval between the two is occupied with the reasoning about Ens and Non-Ens.
[152] Plato, Sophist. p. 263 E. τὰ μὴ ὄντα ὡς ὄντα λεγόμενα, &c.
The distinction between these two propositions, the first as true, the second as false (Theætêtus is sitting down, Theætêtus is flying), is in noway connected with the distinction which Plato had so much insisted upon before respecting the intercommunion of Forms, Ideas, General Notions, &c., that some Forms will come into communion with each other, while others will not (pp. 252-253).