From : Heidegger, Martin: What Is Called Thinking? Harper Perennial. 1976.
The original lecture series was given in 1954: Was heißt Denken?
“We come to know what it means to think when we ourselves try to think. If the attempt is to be successful, we must be ready to learn thinking.
As soon as we allow ourselves to become involved in such learning, we have admitted that we are not yet capable of thinking.
Yet man is called the being who can think, and rightly so. Man is the rational animal. Reason, ratio, evolves in thinking. Being the rational animal, man must be capable of thinking if he really wants to. Still, it may be that man wants to think, but can’t. Perhaps he wants too much when he wants to think, and so can do too little. Man can think in the sense that he possesses the possibility to do so. This possibility alone, however, is no guarantee to us that we are capable of thinking. For we are capable of doing only what we are inclined to do. And again, we truly incline only toward something that in turn inclines toward us, toward our essential being, by appealing to our essential being as the keeper who holds us in our essential being. What keeps us in our essential nature holds us only so long, however, as we for our part keep holding on to what holds us. And we keep holding on to it by not letting it out of our memory.
Memory is the gathering of thought. Thought of what? Thought of what holds us, in that we give it thought precisely because it remains what must be thought about. Thought has the gift of thinking back, a gift given because we incline toward it. Only when we are so inclined toward what in itself is to be thought about, only then are we capable of thinking.
In order to be capable of thinking, we need to learn it first. What is learning? Man learns when he disposes every thing he does so that it answers to whatever essentials are addressed to him at any given moment. We learn to think by giving our mind to what there is to think about.”