To make a discovery is not necessarily the same as to understand a discovery. - Abraham Pais

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.

Of course, relative citation frequencies are no measure of relative importance. Who has not aspired to write a paper so fundamental that very soon it is known to everyone and cited by no one?

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.