A scientist is never certain. ... We absolutely must leave room for doubt or there is no progress and there is no learning.

In physics the truth is rarely perfectly clear, and that is certainly universally the case in human affairs. Hence, what is not surrounded by uncertainty cannot be the truth.

The unanswerable mysteries... the attitude that all is uncertain... to summarize it - the humility of the intellect.

Work as hard and as much as you want to on the things you like to do the best.

If we want to solve a problem that we have never solved before, we must leave the door to the unknown ajar.

But there is nothing in biology yet found that indicates the inevitability of death.

The real problem in speech is not precise language. The problem is clear language.