One must take what nature gives as one finds it.

Thanks to my fortunate idea of introducing the relativity principle into physics, you (and others) now enormously overrate my scientific abilities, to the point where this makes me quite uncomfortable.

The truth of a theory can never be proven, for one never knows if future experience will contradict its conclusions.

If I can't picture it, I can't understand it.

Thinking like we (always) have is what got us where we are. It is not going to get us where we are going.

Not until the creation and maintenance of decent conditions of life for all men are recognized and accepted as a common obligation of all men and all countries — not until then shall we, with a certain degree of justification, be able to speak of mankind as civilized.

Conventional words or other signs have to be sought for laboriously only in a second stage, where the associative play already referred to is sufficiently established and can be reproduced at will.