This is an excerpt from Appendix 8 of “The Age of the World Picture” (Heidegger 2002, 79-80, online at Enowning; see Heidegger 1977, 144-46) on Protagoras’s understanding of being as against Descartes’—which is identical to ours.
Heidegger addresses Protagoras’s “Man [sic] is the measure of all things, of those that are [der seienden], that they are, of those that are not, that they are not” (Pantōn chrēmatōn metron estin anthrōpos, tōn men ontōn hōs estin, tōn de me ontōn hōs ouk estin [cited in Plato’s Theaetetus, 385E ff.]).
He wonders whether this saying implies Plato’s understanding of the Being of a being as an idea (“form, appearance,” from idein, “to see”) and Aristotle’s of our relation to beings as theōria (“seeing,” from theōros, “spectator”)—or even anticipates Descartes’ of the self as subject (the ego cogito, or “[self] thinking self [i.e. as object]”).
He decides, instead, that Plato and Aristotle reacted against the Sophists, and reinterpreted the relation of human beings to beings (while remaining “Greek” in their experience of beings) in a way that came to be identified with Greek thinking and prepared the way for modern interpretations of this relation like Descartes’. (Nonetheless, our humanism prevents us from understanding Being as the Greeks did: in its uniqueness and strangeness, i.e., in a way that cannot but seem unique and strange to us because it’s their way of understanding it—or, perhaps, as something unique and strange, whereas we see it as universal and familiar, viz., matter.)
Understanding what is essential in the fundamental metaphysical positions of Protagoras and Descartes requires that we understand what they say about
- how man is [his Wesensart];
- how beings are [their Wesensauslegung];
- how truth is [its Wesensentwurf];
- how man is measure.
So, to the text . . .
 For Protagoras, to be sure, beings remain related to man as egō [i.e., as I—but not as the ego cogito of Descartes]. Of what kind is this relation to the I? The egō “stays” in [tarries (verweilt) within] the sphere [or horizon] of that which is apportioned to it as this particular unconcealment. Accordingly, it apprehends everything that presences within this sphere as in being [i.e., as a being]. The apprehending of what presences is grounded in this staying [tarrying] within the sphere of unconcealment. The belonging of the I to what presences [i.e., to other beings] is through this staying-alongside what presences. This belonging to what presences in the open draws the boundary between what is present and what absent[s itself]. [+] From out of this boundary man receives and preserves the measure [Maβ as extent] of that which presences and that which absences. In his restriction to that which is unconcealed at a particular time, man gives himself the measure which confines a self in each case to this and that. [-] Man does not set the measure to which all beings in their being here have to accommodate themselves, out of a detached I-ness [as we moderns would have it (after Descartes)].
We are among (“alongside”) beings, i.e. what is present or unconcealed, i.e. not what is absent or concealed. In this way we are the measure of what is and is not. We are not detached from beings, subjects to them as objects, and thereby the measure to which they must conform.
 One who stands in the Greeks’ fundamental relationship to beings and their unconcealment is metron (measure [Mass]), insofar as he accepts restriction [Mässigung] to the sphere of unconcealment limited after the manner of the I [i.e. as an individual]; and, as a consequence, acknowledges the concealment of beings and that their presence or absence, together with the visible appearance of what is present, lies beyond his power of decision [die Unentscheidbarkeit ber dos Aussehen des Was enden]. This is why Protagoras says (Fragment 4 in Diels, Fragmente der Vorsokratiker) Peri men theōn ouk echō eidenai, outh hōs eisin, outh hōs ouk eisin, outh hopoioi tines idean (“Concerning the gods, I am, admittedly, not in a position to know [i.e., for the Greeks, to have something in “sight”] either that they are, or that they are not, nor how they are in their visible aspect [idea]” [“As to the gods, I have no means of knowing either that they exist or that they do not exist”—Nahm]).
Polla gar ta kōluonta eidenai, hē t’adēlotes kai brachus on ho bios tou anthrōpou (“Many, that is, are the things that prevent the apprehending of the being as what it is: both the un-openness [non-disclosure] (concealment) of beings and the brevity of man’s course in history” [“For many are the obstacles that impede knowledge, both the obscurity of the question and the shortness of human life”—Nahm]).
In view of this thoughtful circumspection on Protagoras’ part, it is no wonder that Socrates says of him (Plato,Theaetetus 152b), Eikos mentoi sophon andra mē legein (“We may suppose that he [Protagoras], as a sensible person, was not [in his statement about man as the metron] simply babbling” [“Well, what a wise man says is not likely to be nonsense”—Cornford])
The fundamental metaphysical position of Protagoras is merely a narrowing down—which means, nonetheless, a preserving—of the fundamental position of Heraclitus and Parmenides. Sophism is only possible on the basis of sophia, i.e., on the basis of the Greek interpretation of being as presenc[ing] and truth as unconcealment—an unconcealment which remains itself an essential determination of being, which is why that which presences is determined out of unconcealment, and presencing out of the unconcealed as such.
 But how removed is Descartes from this beginning of Greek thought, how different is the interpretation of man which represents him as subject? In the concept of the subjectum, there still lingers on the sound of the Greek essence of being (the hypokeisqai of the hypokeimenon) in the form of a presencing [the presencing of being] that has become unrecognizable and unquestioned (namely, that which lies permanently at hand). Precisely because of this, we can recognize in this concept of presencing the transformation of the fundamental metaphysical position.
It is one thing to preserve the always limited sphere of unconcealment through the apprehension of what presences (man as metron) [as the Sophists do via the egō]. It is something different to proceed into the unlimited region of possible objectification through the calculating of the representable of which everyone is capable and which is binding on all [as moderns like Descartes do via the subjectum].
Every subjectivism is impossible within Greek Sophism since man can never, here, become subjectum. This cannot happen because, in Sophism, being is presencing and truth is unconcealment. [Here Heidegger goes against Plato, except in the Sophist, and Aristotle.]
[+] In unconcealment, phantasia [“appearance”] happens: the coming[-in]to[-]appearance, as a particular something, of that which presences—for man, who himself presences to what appears [= a mutual presencing—which is at once an “absencing”]. [-] Man as the representing subject fantasizes, however; he moves in imaginatio in that his representation imagines the being as object into the world as picture [= a ].
- [+] phantasia happens; human beings and beings are juxtaposed; human beings by belonging with beings “receive [→ truth] and preserve [→ care]” what is and what is not (happening, juxtaposition, gift)
- [-] human beings represent (fantasizes or imagines) the world as picture; human beings oppose beings; human beings measure what is (picturing, opposition, calculation)
N.B. «Die Zeit des Weltbildes» (9 June 1938) appears in Holzwege (Frankfurt am Main: V. Klostermann, 1963) 69-89; translated as “The Age of the World Picture,” it appears in The Question Concerning Technology and Other Essays, ed. William Lovitt (Harper, 1977) 115-54 (online at Martin Heidegger), and Off the Beaten Track, ed./trans. Julian Young and Kenneth Hynes (Cambridge UP, 2002) 57-85; “The Age of the World View,” trans. Marjorie Grene, Measure 2 (1951): 269-84 and boundary 2 4.2 (Winter 1976) 340-55.