Adversity introduces a man to himself.

One should respect an honest person even if he expresses opinions differing from one's own.

People are like bicycles. They can keep their balance only as long as they keep moving.

Concepts which have proved useful for ordering things easily assume so great an authority over us, that we forget their terrestrial origin and accept them as unalterable facts. They then become labeled as 'conceptual necessities,' etc. The road of scientific progress is frequently blocked for long periods by such errors.

While I am a convinced pacifist there are circumstances in which I believe the use of force is appropriate - namely, in the face of an enemy unconditionally bent on destroying me and my people.

I would absolutely refuse any direct or indirect war service and would try to persuade my friends to do the same, regardless of the reasons for the cause of a war.

... on principle, it is quite wrong to try founding a theory on observable magnitudes alone. In reality the very opposite happens. It is the theory which decides what we can observe.