Now the friends of the Concrete, against whom Plato reasons, used the word _existent_ in a narrower sense, as comprising only the concretes of the sensible world. They probably admitted the existence of the abstract, along with and particularised in the concrete: but they certainly denied the _separate_ existence of the Abstract--_i.e._, of Forms, Attributes, or classes, apart from particulars. They would not deny that many things were conceivable, more or less dissimilar from the realities of the sensible world: but they did not admit that all those conceivable things ought to be termed existent or realities, and put upon the same footing as the sensible world. They used the word _existent_ to distinguish between Men, Horses, Trees, on the one hand--and Cyclopes, Centaurs, [Greek: Trage/laphoi], &c., on the other. A Centaur is just as intelligible and conceivable as either a man or a horse; and according to this definition of Plato, would be as much entitled to be called really existent. The attributes of _man_ and _horse_ are real, because the objects themselves are real and perceivable: the class _man_ and the class _horse_ is real, for the same reason: but the attributes of a Centaur, and the class Centaurs, are not real, because no individuals possessing the attributes, or belonging to the class, have ever been perceived, or authenticated by induction. Plato's Materialist opponents would here have urged, that if he used the word _existent_ or Ens in so wide a sense, comprehending all that is conceivable or nameable, fiction as well as reality--they would require some other words to distinguish fiction from reality--Centaur from Man: which is what most men mean when they speak of one thing as non-existent, another thing as existent. At any rate, here is an equivocal sense of the word Ens--a wider and a narrower sense--which, we shall find frequently perplexing us in the ancient metaphysics; and which, when sifted, will often prove, that what appears to be a difference of doctrine, is in reality little more than a difference of phraseology.[110] [Footnote 110: Plato here aspires to deliver one definition of Ens, applying to all cases. The contrast between him and Aristotle is shown in the more cautious procedure of the latter, who entirely renounces the possibility of giving any one definition fitting all cases. Aristotle declares Ens to be an equivocal word ([Greek: o(mô/numon]), and discriminates several different significations which it bears: all these significations having nevertheless an analogical affinity, more or less remote, with each other. See Aristot. Metaphys. [Greek: D]. 1017, a. 7, seq.; vi. 1028, a. 10. It is declared by Aristotle to be the question first and most disputed in Philosophia Prima, Quid est Ens? [Greek: kai\ dê\ kai\ to\ pa/lai te kai\ nu=n kai\ a)ei\ zêtou/menon kai\ a)ei\ a)porou/menon, tou=to e)sti, ti/s ê( ou)si/a] (p. 1028, b. 2). Compare, B. 1001, a. 6, 31.