In this short lecture, which he gave during his inauguration as professor at Freiburg University in 1928, Heidegger sets himself the task to destroy traditional metaphysics. He starts by asking the question: What is metaphysics? And then immediately decides to not answer this question but to explore the question: What is Nothing? Why? What is the relationship between the metaphysics and Nothing?
We exist, as beings, in a world of beings. When we try to understand what these beings are, we use science. When we try to understand the totality of these beings as such, we use metaphysics. But both science and metaphysics in this way are concerned solely with beings. Hence the central metaphysical question: Why beings and not no-beings? This is delusional, since it leads us away from the starting-point: our existence, as beings concerned with questions of Being, in this world of beings. Our existence in beings (being-in-the-world) is our essence, and by studying our existential modes of being-in-the-world, we can get closer to the only fundamental question: What does Being mean?
Now, when thinking about Being, immediately there pops up its antithesis: Nothing. Traditional metaphysics, since it regards Being as a being itself, views Nothing as a lack of Being. But this is flawed thinking. Nothing, for Heidegger, means no thing. Nothing is not nothing, it is rather the opposite side of Being. This is what Heidegger calls differentation: there's Being and there Nothing, which ultimately are the same. Or, as Heidegger illustratively puts it: Nothingness is Being's veil.
What this means is rather obscure. But it becomes more clear when Heidegger starts claiming even more obscure things, like "Nothing nothings"(das Nichts nichtet). What he means is that Being, as Nothingness, disappears due to our fear of this Nothing. We, as beings which in our being-in-the-world are concerned with Being, suffer from anxiety from this Nothing, the possiblity state of our not-being. Through our apprehension of and mood towards Nothing, we flee into our everyday lives and live as being-with-others (Das Man). So Being disappears from our existence through our fear of Nothing. We, as finite beings fear the infinite Nothing so much that we fall into everyday curiosity, mimicry, and pleasures - we lose sight of the original openness of Being through beings.
For Heidegger, this is rather delusional. We should be ready and prepared to face our fear of Nothing; we should firmly say 'yes!' to enduring in fear; and we should wholly fulfill the claim and pretence Nothing has on us. In short: we should not only accept Nothing as the necessary and most important thing, but we should actively embrace it. Overcoming our fears of Nothing leads us to see that Nothing 'nothings', i.e. it hides the ground of all beings, including us, as Being. And this cures of our tendency to fall for traditional metaphysics which is solely occupied with the totality of beings. In a sense, the whole metaphysical tradition is born out of our cowardly fear of Nothing.
An important consequence of this view of Nothing, being the ground of all beings, is that it becomes the necessary condition that allows denying. So in this sense, Nothing, as the ontological foundation of everything, is the principle that guides our thinking and existence. And this is an outright rejection of logic - which is firmly rooted in - contingently, as it now seems - the principle of non-contradiction.
So it seems Heidegger has, after all, sort-of answered the question 'What is metaphysics?' but in a fascinating and suprising way: it is a mode of inauthentic human existence in which our fear of Nothing makes us flee into the everyday world of beings. Heidegger declares metaphysics to be dead, and any future metaphysics, as it is grounded, as the totality of beings, in beings, impossible and unfruitful.
Of course, this whole lecture is part of the destruction of the philosophical tradition which Heidegger announced in Sein und Zeit (1927) but never published. According to him, the whole western tradition consists of crust upon crust upon crust of inauthentic thinking gone astray. All these layers upon layers of reactions of earlier conceptions of the totality of beings hide the true, original question to us. He perceived himself to be the philosopher that had to destroy this tradition and travel back to the original question - which started it all - 'What does Being mean?' For human beings to be what they are meant to be, we hacve to go back to Ancient Greece and ask the question 'What does Being mean?' anew, and now ask it the right way. Later on in life, Heidegger developed this view into the claim that the qeustion of Being is unanswerable, going as far as to state the question of Being is (literally) waiting a life long...
What we have to make of all this is rather unclear to me. I see where he is coming from, and I understand his central themes and lines of thought. I also can see the beauty in this way of looking at the world and our existence. But I keep wondering if all of Heidegger's philosophy is not rather a fundamentally wrong way to think. I mean, as soon as you throw all the standards of logic overboard, you seem to end up in a vast ocean of interpretation. There is no right or wrong anymore, no criteria by which to judge ideas. Is Heidegger's philosophy, and both phenomenology and existentialism with this, not 'just' an all too human human flaw? We think about our everyday actual experience and end up with conjured up worlds and claims. Shouldn't we just stop with his analysis of our everyday existence? Shouldn't we just leave the question of Being forever alone? Why do things have to have a meaning to begin with? Isn't this way of thinking simply an evolutionary by-product of our tendency to attribute agents and causes to acts and effects?
Anyway, don't read Was ist Metaphysick? (1929) if you haven't read Sein und Zeit (1927) and (preferably) Vom Wesen des Grundes (1928). In Sein und Zeit, the central problem of Being (as being-in-the-world) is worked out, and in Vom Wesen des Grundes Being is tackled. Without these two works, Heideggers treatment of Nothing remains rather...nothing. But it is an important work to read if you want to get a picture of what Heidegger was doing in his earlier carreer.