The reasoning here given from Plato throws some light upon the doctrine just now cited from Antisthenes. You cannot say (argues Plato against the advocates of duality) that two elements (Hot and Cold) are both of them Entia or Existent, because by so doing you call them one. You cannot say (argues Antisthenes) that Sokrates is good, brave, old, &c., because by such speech you call one thing three. Again, in controverting the doctrine of Parmenides, Plato urges, That Ens cannot be Unum, because it is Totum (Unum having no parts, while Totum has parts): but it may carry with it the accident Unum, or may have Unum applied to it as a predicate by accident. Here again, we have difficulties similar to those which perplexed Antisthenes. For the same reason that Plato will not admit, That Ens is Unum — Antisthenes will not admit, That Man is good. It appeared to him to imply essential identity between the predicate and the subject.
All these difficulties and others to which we shall come presently, noway peculiar to Antisthenes — attest the incomplete formal logic of the time: the want of a good theory respecting predication and the function of the copula.
Plato’s argument against the Materialists.
Pursuing the purpose of establishing his conclusion (viz. That Ens involved as many perplexities as Non-Ens), Plato comes to the two opposite sects:— 1. Those (the Materialists) who recognised bodies and nothing else, as the real Entia or Existences. 2. Those (the Friends of Forms, the Idealists) who maintained that incorporeal and intelligible Forms or Species were the only real existences; and that bodies had no existence, but were in perpetual generation and destruction.[102]
[102] Plato, Sophist. p. 246 B.
Respecting the first, Plato says that they must after all be ashamed not to admit, that justice, intelligence, &c., are something real, which may be present or absent in different individual men, and therefore must exist apart from all individuals. Yet justice and intelligence are not bodies. Existence therefore is something common to body and not-body. The characteristic mark of existence is, power or potentiality. Whatever has power to act upon any thing else, or to be acted on by any thing else, is a real Ens or existent something.[103]
[103] Plato, Sophist. p. 247 D-E. λέγω δὴ τὸ καὶ ὁποιανοῦν κεκτημένον δύναμιν, εἴτ’ εἰς τὸ ποιεῖν ἕτερον ὁτιοῦν πεφυκὸς εἴτ’ εἰς τὸ παθεῖν καὶ σμικρότατον ὑπὸ τοῦ φαυλοτάτου, κἂν εἰ μόνον εἰσάπαξ, πᾶν τοῦτο ὄντως εἶναι· τίθεμαι γὰρ ὅρον ὁρίζειν τὰ ὄντα, ὡς ἔστιν οὐκ ἄλλο τι πλὴν δύναμις.
Reply open to the Materialists.
Unfortunately we never know any thing about the opponents of Plato, nor how they would have answered his objection — except so much as he chooses to tell us. But it appears to me that the opponents whom he is here confuting would have accepted his definition, and employed it for the support of their own opinion. “We recognise (they would say) just men, or hard bodies, as existent, because they conform to your definition: they have power to act and be acted upon. But justice, apart from just men — hardness, apart from hard bodies — has no such power: they neither act upon any thing, nor are acted on by any thing: therefore we do not recognise them as existent.” According to their view, objects of perception acted on the mind, and therefore were to be recognised as existent: objects of mere conception did not act on the mind, and therefore had not the same claim to be ranked as existent: or at any rate they acted on the mind in a different way, which constitutes the difference between the real and unreal. Of this difference Plato’s definition takes no account.[104]
[104] Plato, Sophist. p. 247 E. τὸ καὶ ὁποιανοῦν κεκτημένον δύναμιν, &c.