The one main theme intended for examination here (as Sokrates[12] expressly declares) is the doctrine--That Cognition is sensible perception. Nevertheless upon all the three opinions, thus represented as cognate or identical,[13] Sokrates bestows a lengthened comment (occupying a half of the dialogue) in conversation, principally with Theætêtus, but partly also with Theodôrus. His strictures are not always easy to follow with assurance, because he often passes with little notice from one to the other of the three doctrines which he is examining: because he himself, though really opposed to them, affects in part to take them up and to suggest arguments in their favour: and further because, disclaiming all positive opinion of his own, he sometimes leaves us in doubt what is his real purpose--whether to expound, or to deride, the opinions of others--whether to enlighten Theætêtus, or to test his power of detecting fallacies.[14] We cannot always distinguish between the ironical and the serious. Lastly, it is a still greater difficulty, that we have not before us either of the three opinions as set forth by their proper supporters. There remains no work either of Protagoras or of Herakleitus: so that we do not clearly know the subject matter upon which Plato is commenting--nor whether these authors would have admitted as just the view which he takes of their opinions.[15] [Footnote 12: Plato, Theætêt. p. 163 A.] [Footnote 13: Plato, Theætêt. p. 160 D. _Sokrat._ [Greek: Pagka/lôs a)/ra soi ei)/rêtai o(/ti e)pistê/mê ou)k a)/llo ti/ e)stin ê)\ ai)/sthêsis; kai\ _ei)s tau)to\n sumpe/ptôke_, kata\ me\n O(/mêron kai\ Ê(ra/kleiton kai\ pa=n to\ toiou=ton phu=lon, oi(=on r(eu/mata kinei=sthai ta\ pa/nta--kata\ de\ Prôtago/ran to\n sophô/taton, pa/ntôn chrêma/tôn a)/nthrôpon me/tron ei)=nai--kata\ de\ Theai/têton, tou/tôn ou(=tôs e)cho/ntôn, ai)/sthêsin e)pistê/mên gi/gnesthai.]] [Footnote 14: See the answer of Theætêtus and the words of Sokrates following, p. 157 C.] [Footnote 15: It would be hardly necessary to remark, that when Plato professes to put a pleading into the mouth of Protagoras (pp. 165-166) we have no other real speaker than Plato himself, if commentators did not often forget this. Steinhart indeed tells us (Einleit. zum Theætêt. pp. 36-47) positively--that Plato in this pleading keeps in the most accurate manner (auf das genaueste) to the thoughts of Protagoras, perhaps even to his words. How Steinhart can know this I am at a loss to understand. To me it seems very improbable. The mere circumstance that Plato forces into partnership three distinct theories, makes it probable that he did not adhere to the thoughts or language of any one of them.] [Side-note: The doctrine of Protagoras is completely distinct from the other doctrines. The identification of them as one and the same is only constructive--the interpretation of Plato himself.]