One of the absolute rules I learned in the war was, don't know anything you don't need to know, because if you ever get caught they will get it out o… - Abraham Pais

The rule of the game was never assume that anybody, however honorable, would be able to stand up under torture. If Mr. X, who knew where I was, was caught for some reason, I should move.

One of the things I learned, one of the strangest things, is how to think. There was nothing else to do. I couldn't see people, or go for a walk in the forest. All I had was my head and my books, and I thought a lot. I learned, because there was no interruption. I had access to myself, to my thinking. I wouldn't say that I particularly matured. The thinking was physics thinking. I was just short of twenty-two then. I was in hiding for two years and two months, something like that. In all that time I went out very, very little, just once in a great while, after dark. Once I even took the train to Utrecht, forty miles from Amsterdam, with my yellow star, this star which I still have. Why did I go? I just wanted to visit some friends. I was a little bit crazy, a little bit insane.

He understood that the sources of the gravitational field were not just ponderable matter but also field energy. He realized that gravitational field energy is to be included as a source and that the gravitational field equations were therefore bound to be nonlinear.