To pass over this inconsistency, however--the reasoning of Plato himself on the subject of Non-Ens is an interesting relic of ancient speculation. He has made for himself an opportunity of canvassing, not only the doctrine of Parmenides, who emphatically denied Non-Ens--but also the opposite doctrine of other schools. He farther comments upon a different opinion, advanced by other philosophers--That no proposition can be admitted, in which the predicate is different from the subject: That no proposition is true or valid, except an identical proposition. You cannot say, Man is good: you can only say, Man is Man, or Good is good. You cannot say--Sokrates is good, brave, old, stout, flat-nosed, &c., because you thereby multiply the one Sokrates into many. One thing cannot be many, nor many things one.[100] [Footnote 100: Plato, Sophist. p. 251 B-C. Compare Plato, Philêbus, p. 14 C.] [Side-note: Misconception of the function of the copula in predication.] This last opinion is said to have been held by Antisthenes, one of the disciples of Sokrates. We do not know how he explained or defended it, nor what reserves he may have admitted to qualify it. Plato takes no pains to inform us on this point. He treats the opinion with derision, as an absurdity. We may conceive it as one of the many errors arising from a misconception of the purpose and function of the copula in predication. Antisthenes probably considered that the copula implied identity between the predicate and the subject. Now the explanation or definition of _man_ is different from the explanation or definition of _good_: accordingly, if you say, Man is good, you predicate identity between two different things: as if you were to say Two is Three, or Three is Four. And if the predicates were multiplied, the contradiction became aggravated, because then you predicated identity not merely between one thing and another different thing, but between one thing and many different things. The opinion of Antisthenes depends upon two assumptions--That each separate word, whether used as subject or as predicate, denotes a Something separate and existent by itself: That the copula implies identity. Now the first of these two assumptions is not unfrequently admitted, even in the reasonings of Plato, Aristotle, and many others: while the latter is not more remarkable than various other erroneous conceptions which have been entertained, as to the function of the copula. [Side-note: No formal Grammar or Logic existed at that time. No analysis or classification of propositions before the works of Aristotle.]