What is most important to observe is--That at the time which we are here discussing, there existed no such sciences either grammar or formal logic. There was a copious and flexible language--a large body of literature, chiefly poetical--and great facility as well as felicity in the use of speech for the purposes of communication and persuasion. But no attempt had yet been made to analyse or theorise on speech: to distinguish between the different functions of words, and to throw them into suitable classes: to generalise the conditions of good or bad use of speech for proving a conclusion: or to draw up rules for grammar, syntax, and logic. Both Protagoras and Prodikus appear to have contributed something towards this object, and Plato gives various scattered remarks going still farther. But there was no regular body either of grammar or of formal logic: no established rules or principles to appeal to, no recognised teaching, on either topic. It was Aristotle who rendered the important service of filling up this gap. I shall touch hereafter upon the manner in which he proceeded: but the necessity of laying down a good theory of predication, and precepts respecting the employment of propositions in reasoning, is best shown by such misconceptions as this of Antisthenes; which naturally arise among argumentative men yet untrained in the generalities of grammar and logic. [Side-note: Plato's declared purpose in the Sophistês--To confute the various schools of thinkers--Antisthenes, Parmenides, the Materialists, &c.] Plato announces his intention, in this portion of the Sophistês, to confute all these different schools of thinkers, to whom he has made allusion.[101] His first purpose, in reasoning against those who maintained Non-Ens to be an incogitable absurdity, is, to show that there are equal difficulties respecting Ens: that the Existent is just as equivocal and unintelligible as the Non-Existent. Those who recognise two co-ordinate and elementary principles (such as Hot and Cold) maintain that both are really existent, and call them both, Entia. Here (argues Plato) they contradict themselves: they call their two elementary principles _one_. What do they mean by existence, if this be not so? [Footnote 101: Plato, Sophist. p. 251 C-D. [Greek: I(/na toi/nun pro\s a(/pantas ê(mi=n o( lo/gos ê)=| tou\s pô/pote peri\ ou)si/as kai\ o(tiou=n dialechthe/ntas, e)/stô kai\ pro\s tou/tous kai\ pro\s tou\s a)/llous, o(/sois e)/mprosthen dieile/gmetha, ta\ nu=n ô(s e)n e)rôtê/sei lechthêso/mena.]]