Instead we must say: Ἀλήθεια, as opening of presence and presenting in thinking and saying, originally comes under the perspective of ὁμοίωσις and adaequatio, that is, the perspective of adequation in the sense of the correspondence of representing with what is present.

But this process inevitably provokes another question: How is it that ἀλήθεια, unconcealment, appears to man's natural experience and speaking only as correctness and dependability? Is it because man's ecstatic sojourn in the openness of presencing is turned only toward what is present and the existent presenting of what is present? But what else does this mean than that presence as such, and together with it the opening granting it, remain unheeded? Only what ἀλήθεια as opening grants is experienced and thought, not what it is as such.

This remains concealed. Does this happen by chance? Does it happen only as a consequence of the carelessness of human thinking? Or does it happen because self-concealing, concealment, λήθη belongs to ἀ-λήθεια, not just as an addition, not as shadow to light, but rather as the heart of ἀλήθεια? And does not even a keeping and preserving rule in this self-concealing of the opening of presence from which unconcealment can be granted to begin with, and thus what is present can appear in its presence?

If this were so, then the opening would not be the mere opening of presence, but the opening of presence concealing itself, the opening of a self-concealing sheltering.

If this were so, then with these questions we would reach the path to the task of thinking at the end of philosophy.

But isn't this all unfounded mysticism or even bad mythology, in any case a ruinous irrationalism, the denial of ratio?

I return to the question: What does ratio, νοῦς, νοεῖν, perceiving (Vernunft—Vernehmen) mean? What does ground and principle and especially principle of all principles mean? Can this ever be sufficiently determined unless we experience ἀλήθεια in a Greek manner as unconcealment and then, above and beyond the Greek, think it as the opening of self-concealing? As long as ratio and the rational still remain questionable in what is their own, talk about irrationalism is unfounded.

On Time and Being (GA 14) by Martin Heidegger