Only the [Catholic] Church stood squarely across the path of Hitler’s campaign for suppressing truth. I never had any special interest in the Church before, but now I feel a great affection and admiration because the Church alone has had the courage and persistence to stand for intellectual truth and moral freedom. I am forced thus to confess that what I once despised I now praise unreservedly.

Those instrumental goods which should serve to maintain the life and health of all human beings should be produced by the least possible labour of all.

[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.

Conviction is a good motive, but a bad judge.

Pure mathematics is, in its way, the poetry of logical ideas.

I never teach my pupils, I only attempt to provide the conditions in which they can learn.

[Freud's] sense of reality is less clouded by wishful thinking than is the case with other people and [he combines] the qualities of critical judgment, earnestness and responsibility.