It was about three o'clock at night when the final result of the calculation [which gave birth to quantum mechanics] lay before me ... At first I was deeply shaken ... I was so excited that I could not think of sleep. So I left the house ... and awaited the sunrise on top of a rock.

...separation of the observer from the phenomenon to be observed is no longer possible.

The exact sciences also start from the assumption that in the end it will always be possible to understand nature, even in every new field of experience, but that we may make no a priori assumptions about the meaning of the word understand.

A consistent pursuit of classical physics forces a transformation in the very heart of that physics.

It will never be possible by pure reason to arrive at some absolute truth.

Whether we electrons, light quanta, benzol molecules, or stones, we shall always come up against these two characteristics, the corpuscular and the undular.

Every tool carries with it the spirit by which it has been created.