[Footnote 21: See this point handled in Sextus Empiric. adv. Mathemat. viii. 355-362. We may here cite a remark of Simplikius in his Commentary on the Categories of Aristotle (p. 64, a. in Schol. Brandis). Aristotle (De Animâ, iii. 2, 426, a. 19; Categor. p. 7, b. 23) lays down the doctrine that in most cases Relata or ([Greek: ta\ pro/s ti]) are "simul Naturâ, [Greek: kai\ sunanairei= a)/llêla]": but that in some Relata this is not true: for example, [Greek: to\ e)pistêto\n] is relative to [Greek: e)pistê/mê], yet still _it would seem prior_ to [Greek: e)pistê/mê (pro/teron a)\n do/xeie tê=s e)pistê/mês ei)=nai)]. There cannot be [Greek: e)pistê/mê] without some [Greek: e)pistêto/n]: but there may be [Greek: e)pistêto\n] without any [Greek: e)pistê/mê]. There are few things, if any (he says), in which the [Greek: e)pistêto\n] (cognoscibile) is _simul naturâ_ with [Greek: e)pistê/mê] (or cognitio) and cannot be without it. Upon which Simplikius remarks, What are these few things? [Greek: Ti/na de\ ta\ o)li/ga e)sti/n, e)ph' ô(=n a(/ma tô=| e)pistêtô=| ê( e)pistê/mê e)sti/n? Ta\ a)/neu u(/lês, ta\ noêta/, a(/ma tô=| kat' e)nergei/an a)ei\ e)stô/sê| e)pistê/mê e)/stin, ei)/te kai\ e)n ê(mi=n e)sti/ tis toiau/tê a)ei\ a)/nô me/nousa, . . . ei)/te kai\ e)n tô=| kat' e)nergei/an vô=| ei)/ tis kai\ tê\n no/êsin e)kei/nên e)pistê/mên e(/loito kalei=n. du/natai de\ kai\ dia\ tê\n tô=n koinô=n u(po/stasin ei)rê=sthai, tê\n e)x a)phaire/seôs; a(/ma ga\r tê=| u(posta/sei tou/tôn kai\ ê( e)pistê/mê e)sti/n. a)lêthe\s de\ kai\ e)pi\ tô=n a)naplasma/tôn tô=n te e)n tê=| phantasi/a| kai\ tô=n technitô=n; a(/ma ga\r chi/maira kai\ ê( e)pistê/mê chimai/ras.] We see from hence that Simplikius recognises Concepts, Abstractions, and Fictions, to be dependent on the Conceiving, Abstracting, Imagining, Mind--as distinguished from objects of Sense, which he does not recognise as dependent in the like manner. He agrees in the doctrine of Protagoras as to the former, but not as to the latter. This illustrates what I have affirmed, That the Protagorean doctrine of "_Homo Mensura_" is not only unconnected with the other principle (that Knowledge is resolvable into sensible perception) to which Aristotle and Plato would trace it--but that there is rather a repugnance between the two. The difficulty of proving the doctrine, and the reluctance to admit it, is greatest in the case of material objects, least in the case of Abstractions, and General Ideas. Yet Aristotle, in reasoning against the Protagorean doctrine (Metaphysic. [Greek: G]. pp. 1009-1010, &c.) treats it like Plato, as a sort of corollary from the theory that Cognition is Sensible Perception.