Openness is not only the condition of the possibility of the correctness of an assertion. As such a condition it appears for the first time only in the subsequent critical reference. But to be such a condition does not exhaust the essence of openness, nor does it touch the heart of this essence. For openness expresses something even more original than ἀλήθεια, not only the unconcealedness of present beings, but also what is illuminated in the clearing and the clearing itself, in which an unconcealed being can stand forth in the first place.

What is this clearing in the midst of beings? What must it be, so that in it beings can encounter and belong to one another? Where is its ground and how does this illuminated "in the midst" come to presence, into which man is displaced by disposition and which he has to occupy and preserve in the forbearance of his creative activity? The openness of the illuminated "in-between," in which man comes to stand, reveals itself in this way as the ground of humanity itself—not of some sort of universal humanity, but of that man who by means of the question of the essence of truth as openness first raises the question of who he is. In our retrospective sketch of the beginning of Western thinking, we said that man was determined there as the custodian of the unconcealedness of beings and later declined into the rational animal. In asking about the more original essence of truth as the openness of beings, the question of who man is first attains its keen edge and its necessity. For this question now asks whether man really is the steward of the essence of truth and whether all his truths and correctnesses do not remain fragmentary and preliminary, as long as and as often as he forgets this stewardship.

The essence of openness is not exhausted there but is more original. That is the reference of what was said about disposition and its dislocating and casting asunder of beings.

Openness is not only what makes this possible—i.e., a particular human comportment, the predicating and judging about objects—but is what makes man himself possible in the first place, insofar as he is finally and genuinely understood in terms of that which his Western history primordially throws him into, in order that, as it seems, at first he would not grasp it but would only disfigure it by forgetting it.

And what is this? The fact that man is not only—as we interpreted him in our retrospective sketch—the preserver of the unconcealedness of beings but is the steward of the openness of Being itself, in who~se~ play of space and time beings first come to be beings (more so and less). Then this would be the decision of future mankind and the preparation of the present, that man of today might overcome himself and his truth. and instead of continuing on, i.e., continuously treading in the same place, might find His essence out of a more original ground and begin to become that essence—namely, the guardian of the truth of Being.

Openness comes to pass as die clearing of self-concealment, as the "there" [Da] in the grounding-there [Da-gründung] of being-the-there [Da-sein].

Basic Questions of Philosophy (GA 45) by Martin Heidegger

GA 45 p. 226