Dense, obtuse, crazy. I am left with more questions than answers. Science, with its infinite ability to categorize: "not this but that," or "this is not that," or "this, because it is not something else, is what it is." Science is the one who is being brought into question by Heidegger. And by this?
Yes, what sort of an address is this? Who of his original Freiburg hearers would have come close to understanding it? When I read it to myself or to others, it sounds like the babblings of an idiot. Yet, its edge is placed at the deepest root of a great and monumental tree. This is a funeral oration. "Metaphysics is dead, long live metaphysics!"
As far as I understand it now, the principle problem with classical metaphysics is that it has treated its "not" as something over and against its "is" - antimatter vs matter, yet both are substantives. This is incorrect, says Heidegger. "The nothing is the origin of negation, not vice versa." The nothing is an abyss that does not exist as something among something, but as a nothing in which everything is "held out" (suspended?) Therefore, classical metaphysics has got it all wrong, and so, for that matter, has science which is itself build on Aristotelian foundations. Both of these disciplines need to take a humble pill and return to their original source, which is the exploration of truth.
The way that Heidegger achieves this stripping of the altars in this address is almost comic. His language sounds more like comic impersonation than real philosophy, tempting one to simply make fun of its incomprehensibilities and ignore the real argument. But don't think for a second that there isn't a real argument going on. Look, for example, at the following:
"The more we turn toward beings in our preoccupations the less we let beings as a whole slip away as such and the more we turn away from the nothing. Just as surely do we hasten into the public superficies of existence. And yet this constant if ambiguous turning away from the nothing accords, within certain limits, with the most proper significance of the nothing. In its nihilation the nothing directs us precisely toward beings. The nothing nihilates incessantly without our really knowing of this occurrence in the manner of our everyday knowledge."
Heidegger the iconoclast soothes the ego of his scientist even as he defrocks him!
Incidentally, the text I have used here (Peter Krell, 1977) is not as good as several other translations available on the web, most notably that made by Thomas Sheehan of Stanford (2001). I am tempted to delete everything and start over...