Protagoras was a Sophist and agnostic, a contemporary of Socrates in Athens, whose saying that man is the measure of all things is discussed in Plato's Theaetetus. He serves as the origin of moral relativism and human centered metaphysics. As such he has been used as a point of departure for discussion by many philosophers, including Heidegger. How Protagoras, and other philosophers who commented on him, are understood can help us to understand Heidegger and distinguish his way of thinking.

Appendix eight to The Age of the World Picture is a discussion of Protagoras that situates his relativism and agnosticism in the context of Greek thinking.

But did not a Sophist at about the time of Socrates venture to say that "Man is the measure of all things, of what are, that they are, of what are not, that they are not?" Does not this statement of Protagoras sound as though it were Descartes speaking? Is it not through Plato that the being of beings if fully grasped as the visible, the idea? Is not the relation to beings as such, for Aristotle, pure looking? And yet it is no more the case the Protagoras' Sophistic statement is subjectivism than it is the case that Descartes had the capacity to bring about the overturning of Greek thought. Through Plato's thinking and Aristotle's questioning there occurred, to be sure, a decisive transformation of the interpretation of beings and of man. But this transformation always remained within the Greeks' fundamental experience of beings. Precisely as a struggle against the Sophistic, and so as dependent on it, this transformed interpretation proves so decisive as to become the ending of the Greek world, and ending which indirectly helps to prepare the possibility of the modern age. This is the reason that, later on, not just in the Middle Ages but right through the modern age and up to the present, Platonic and Aristotelian thought was able to be taken as Greek thought per se, and why all pre-Platonic thought could be considered to be merely a preparation for Plato. Because we have long been accustomed to understand Greece in terms of a modern humanistic interpretation, it remains denied to us to think being as it opened itself to Greek antiquity, to think it in a way that allows it its ownness and strangeness.

Protagoras' statement reads

pantwn crhmatwn metron estin anqrwpoV, twn men ontwn wV esti, twn de mh ontwn wV ouk estin.
(cf. Plato's Theaetetus 153a)

Of all things (those, namely, that man has around him in use and usage, crhmata crhsqai) man is (in each case) the measure, of what presences, that it so presences, of that, however, to which presencing is denied, that it does not presence.

The being whose being is up for decision is understood, here, as that which is present in the sphere of man, arriving in this region, of itself. Who, however, is "man"? Plato tells us in the same passage by having Socrates say:

Does he (Protagoras) not understand this somewhat as follows? Whatever, at a given time, something shows itself to me as, of such an aspect is it (also) for me; but whatever it shows itself to you as, is it not such a turn for you? But you are a man just as much as I.

Man is here, accordingly, the man in each particular case (I and you, he and she). And should not this egw coincide with Descartes' ego cogito? Never. For in every essential respect, what determines the two fundamental metaphysical positions with equal necessity is different. What is essential to a fundamental metaphysical position embraces:

(1) The manner and way in which man is man, that is, himself: the essential nature of selfhood which by no means coincides with I-ness, but is rather determined by the relationship to being as such.
(2) The essential interpretation of the being of beings.
(3) The essential projection of truth.
(4) The sense in which, in any given instance, "man is the measure."

None of the essential moments of the fundamental metaphysical position can be understood apart from the others. Each, by itself, indicates the totality of a fundamental metaphysical position. For what reason, and to what extent, just these four moments bear and structure a fundamental metaphysical position in advance is a question which can no longer be asked or answered out of or through metaphysics. To ask it is already to speak out of the overcoming of metaphysics.

[Continued]  ¶ 8:38 AM