You can never solve a problem on the level on which it was created. Insanity: doing the same thing over and over again and expecting different results. A life lived for others is a life worthwhile.

[Kepler] had to realize clearly that logical-mathematical theoretizing, no matter how lucid, could not guarantee truth by itself; that the most beautiful logical theory means nothing in natural science without comparison with the exactest experience. Without this philosophic attitude, his work would not have been possible.

Intuitive powers played a central role in my scientific work, not wild speculation, yet a valued resource when no other approach was available.

The distinction between the past, present and future is only a stubbornly persistent illusion.

Small is the number of them that see with their own eyes and feel with their own hearts.

There is no greater satisfaction for a just and well-meaning person than the knowledge that he has devoted his best energies to the service of the good cause.

The individual feels the futility of human desires and aims and the sublimity and marvelous order which reveal themselves both in nature and in the world of thought.