It is not improbable that the three doctrines, here put together by Plato and subjected to a common scrutiny, may have been sometimes held by the same philosophers. Nevertheless, the language[16] of Plato himself shows us that Protagoras never expressly affirmed knowledge to be sensible Perception: and that the substantial identity between this doctrine, and the different doctrine maintained by Protagoras, is to be regarded as a construction put upon the two by Plato. That the theories of Herakleitus and Empedokles differed materially from each other, we know certainly: the theory of each, moreover, differed from the doctrine of Protagoras--"Man is the measure of all things". How this last doctrine was defended by its promulgator, we cannot say. But the defence of it noway required him to maintain--That knowledge is sensible perception. It might be consistently held by one who rejected that definition of knowledge.[17] And though Plato tries to refute both, yet the reasonings which he brings against one do not at all tell against the other. [Footnote 16: See Theætêt. p. 152 A. This is admitted (to be a construction put by Plato himself) by Steinhart in his note 7, p. 214, Einleitung zum Theætêtus, though he says that Plato's construction is the right one.] [Footnote 17: Dr. Routh, in a note upon his edition of the Euthydêmus of Plato (p. 286 C) observes:--"Protagoras docebat, [Greek: Pa/ntôn chrêma/tôn me/tron a)/nthrôpon ei)=nai, tô=n me\n o)/ntôn, ô(s e)/sti; tô=n de\ mê\ o)/ntôn, ô(s ou)k e)/sti.] Quâ quidem opinione qualitatum sensilium sine animi perceptione existentiam sustulisse videtur." The definition here given by Routh is correct as far as it goes, though too narrow. But it is sufficient to exhibit the Protagorean doctrine as quite distinct from the other doctrine, [Greek: o(/ti e)pistê/mê ou)k a)/llo ti/ e)stin ê)\ ai)/sthêsis.]] [Side-note: Explanation of the doctrine of Protagoras--_Homo Mensura_.]