There is no learning without having to pose a question. And a question requires doubt.

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

There is enough energy in a single cubic meter of space to boil all the oceans in the world.

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

Philosophers say a great deal about what is absolutely necessary for science, and it is always, so far as one can see, rather naive, and probably wrong.

There are thousands of years in the past, and there is an unknown amount of time in the future. There are all kinds of opportunities, and there are all kinds of dangers.

Turbulence is the most important unsolved problem of classical physics.