The theory is now fully stated, and Socrates moves on to the criticism of it. He first makes some rather trivial objections. He points out, in the first place, that the theory does not justify taking humanity as the measure of all things. With just as good reason, one might take a pig or a horse as the measure, if knowledge is merely an interaction between a flux and a private sense organ. The criticism is not developed seriously; nevertheless, there is a purpose in making it. It suggests, without explicitly stating, a crucial element in the discussion of knowing; namely, that a judging mind is involved wherever there is a genuine case of knowing. A judging mind is precisely what a pig or horse lacks; thus, it is ridiculous to say that a pig is the measure of all things. The other criticisms Socrates makes at this time also imply the same point, that a judging mind must be included in any theory of knowing.

Three additional criticisms are made here:1. The theory, because it rejects any common world shared by two knowers, provides no justification for Protagoras’s life as a teacher. Protagoras cannot justify his role as a teacher who corrects his pupils unless he is in some sense the measure of his pupils’ worlds. 2. Sensations cannot be all there is to knowing because this would imply that one who cannot read would nevertheless know what is said on a page of writing when he merely sees it. Yet this clearly is not the case. 3. The theory would require one to deny that people who close their eyes know what they have just seen.

Socrates next undertakes to defend Protagoras against these criticisms. He states that Protagoras would regard the criticisms as irrelevant because what should be refuted is either the claim that people’s sensations are private or the claim that the object causing the sensation is private. Still speaking on behalf of Protagoras, Socrates adds that the earlier criticisms missed the point and spirit of Protagoras’s position.

Protagoras could offer a justification of his teaching by pointing to the analogous case of a physician. The physician does not deny the reality of his patient’s (distorted) world; instead, he changes the condition of the patient so that the patient’s world is changed (thus losing its distortion). The wine really is bitter to a sick person, but the physician makes the patient well so that the same wine tastes sweet to the recovered individual. (In a sense, this is to say that the sick Socrates is not the same man as the well Socrates.) We can admit that the wine is both bitter and sweet without thereby destroying the right of the physician to perform his beneficial work.

This justification, however, rests on restricting our concern to what is useful rather than on raising the question of what is true. Questions of truth concern the external object of knowledge; questions of utility can be restricted to the state of the knower. Protagoras, as teacher, can justify modifying the state of the knower without rejecting his claim that the object is a part of a private world in which there is no difference between what appears and what is. The earlier criticism has not touched Protagoras’s real claim; namely, that which seems or appears to be true for an individual knower is true for that knower.