הטיעון הדיאלקטי נגד פרוטגורס (או רלטיביזם בכלל) הוא כדלקמן: אם הדברים הם כפי שהם נראים לכל אדם, באופן פרטי (או אם הטענה, לפיה, האדם הוא ההומו מז׳רה אוף אול פינגס, היא נכונה), אז לא יהיו לנו אנשים חכמים מחד גיסא, ואנשים בורים מאידך גיסא. אבל לא השני; ולכן לא הראשון. 385E4-386D7 Cratylus
SOC: Come, then, Hermogenes, let’s see whether entities seem to you to be disposed like this – that their essence is a private matter for each individual – as Protagoras claimed when he said that man is the ‘measure of all things’. So, things are for me however they appear to me, and for you however they appear to you. Or do they seem to you to have some stable essence of their own?
HERM: Occasionally, Socrates, when confounded, I have been carried away to this view of Protagoras. But he doesn’t seem to me to be entirely right.
SOC: What, then? Were you carried away so far that no man seems to you to be bad?
HERM: No, by Zeus! But many times I have felt that certain men are quite bad, very many indeed.
SOC: What, then? Haven’t any seemed to you quite good?
HERM: Very few indeed.
SOC: So they did seem good?
HERM: Yes, they did.
SOC: How, then, do you assert this? As follows, that the very good are the very wise, but the very bad are the very foolish?
HERM: It seems so.
SOC: So, if Protagoras was speaking the truth, and it is true that things really are as they seem to each of us, is it possible that some of us be wise but others foolish?
HERM: Of course not.
SOC: And, as I see it, at least this seems certain to you, that, since there is wisdom and foolishness, it is not at all possible that Protagoras is speaking the truth. For I suppose, one would really be no wiser than another, if whatever seems so for each person is true for each.
HERM: This is so.