As far as the laws of mathematics refer to reality, they are not certain, and as far as they are certain, they do not refer to reality.

The truth of a theory is in your mind, not in your eyes.

I am not a positivist. Positivism states that what cannot be observed does not exist. This conception is scientifically indefensible, for it is impossible to make valid affirmations of what people 'can' or 'cannot' observe. One would have to say 'only what we observe exists,' which is obviously false.

A theory is the more impressive the greater the simplicity of its premises is, the more different kinds of things it relates, and the more extended its area of applicability.

I never think of the future. It comes soon enough. When visiting the U.S. from Germany for a winter academic stay.

A tyranny based on ... deception and maintained by terror must inevitably perish from the poison it generates within itself.

Only the Catholic Church protested against the Hitlerian onslaught on liberty. Up till then I had not been interested in the Church, but today I feel a great admiration for the Church, which alone has had the courage to struggle for spiritual truth and moral liberty