This explanation requires a quick romp through Heraclitus, Heidegger, and Eckhart. Aletheia - the Greek notion of truth that Heidegger favors - according to Heraclitus is like a flash of lightning (Lichtung in Heidegger's German evaluation). This illumination reveals itself by making known things in their "thingness," yet conceals itself by redirecting our focus away from the truth of Being (the flash), towards the quiddity of a being (that which is illuminated by the flash). We see not the lightning itself, but the trees, rocks, etc that the brief flash illuminates. John D. Caputo, in Heidegger and Aquinas puts it this way,
"The mistake of mortals, against which Heraclitus warns, is to turn from the clearing as such and in the clearing, to turn away from presence in favor of what is present...
The Greeks experienced logos [the organization of the gathering into presence that aletheia reveals] but never thought it, and that includes Heraclitus. They allowed Being to come into language as the difference between Being and beings, but they did not name that difference as the essence of language."Essentially, the common mistake is to miss the forest for the trees, to see what is illuminated rather than the illumination itself and mistake that for true Being. Heidegger calls this the "oblivion" that we must overcome by thinking Being.
For a while I tried very hard to "think Being," to turn my eyes to the lightning itself and overcome the oblivion. I tried very much to see things not as things, but as the result of a revelatory act of Being, and fixate on that Being itself. But I soon realized I was way off base - at best misguided in my methods.
I was trying too hard, becoming too attached to the object in hopes that it might transfigure before my eyes into something strange. But any diligent mystic could quickly catch my error, citing Eckhart's insistence on Detachment.
Detachment is the "letting-be" described by Heideggerian gelassenheit. Again I defer to Caputo:
"In gelassenheit we put ourselves at the disposal of the matter and no longer attempt to reign over it with our concepts. In our surrendering to it, it alone holds sway; the matter is not seized about, but let-be and hence preserved in its inexhaustibility... lethe [forgetfulness/concealment - in this case the preservation of the unknowing of the object that goes hand in hand with the a-lethic revelation] is lost therefore the moment that thinking detaches itself from the thing and assumes the posture of an onlooker, of the judge and jury of the 'object.'"A quick note on the dialectic of lethe/aletheia: It is important that the two go hand in hand, knowing and unknowing - the object must be both found and lost, revealed and concealed. If one were to only gain knowledge (terminating lethe) then the contemplative act would cease because our knowledge of the object would cease to be subservient to the object's Being, causing us to lose sight of Being and fall back into Oblivion. And if one were to revel in lethe (never reaching aletheia), one would never see the being in its Being, remaining in Oblivion.
This exercise becomes a mystical one when the "object" of our gaze becomes not a tree or a rock, but God him/her/itself. The overcoming of Heideggerian Oblivion is actually the retention of the Oblivion. We come into the knowledge of Being only to lose our knowledge of the being, which is in itself an instance of Being (and God may or may not even be Being itself, depending on who you ask). Our gaze is cast upon God, to see Being in the Lichtung and consequentially know less about it than when we started. We must let God be, clearing ourselves in an act of faithful gelassenheit. This gives the object, God, free reign to Be above and beyond our expectations and understanding. Perhaps we may even catch a glimpse of it.
May we suspend our propensities for judgement and control, that we might encounter divine Being and unknow it all the more.