ABSTRACT
Plato has now shown that insofar as Theaetetus’ definition of knowledge as perception coincides with and draws support from the Protagorean doctrine of man the measure, it is not acceptable. As soon as Protagoreanism is extended beyond the judgments which each man makes about the sense-contents actually present to his consciousness at the time of speaking (179c 1–4), and made to apply to judgments about future events, what is advantageous, etc., it is quite certainly a false doctrine. Protagoreanism, as a distinctive theory, must be rejected. If any part of it is to be retained it will be its application to judgments of immediate sense-perception (cf. 171e 1–3). But, now that as a general doctrine Protagoreanism has been refuted, immediate sense perception must be examined on its own ground to see whether it is always true and whether its judgments are always known to be true. If it turns out that they are known, then to this extent Protagoreanism will have been vindicated; but one cannot any longer reason in the other direction and use Protagoreanism to vindicate sense-perception’s claim to be knowledge.
