My rule is, when you are unhappy, think about it. But when you're happy, don't. Why spoil it? You're probably happy for some ridiculous reason and you'd just spoil it to know it.

There was a time when the newspapers said that only twelve men understood the theory of relativity. I do not believe there ever was such a time ... On the other hand, I think I can safely say that nobody understands quantum mechanics.

When you get as old as I am, you start to realize that you've told most of the good stuff you know to other people anyway.

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

The electron is a theory we use; it is so useful in understanding the way nature works that we can almost call it real.

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

The highest forms of understanding we can achieve are laughter and human compassion.