I don't believe in astrology; I'm a Sagittarius and we're skeptical.

That's one of those meaningless and unanswerable questions the mind keeps returning to endlessly, like the tongue exploring a broken tooth.

I don't pretend we have all the answers. But the questions are certainly worth thinking about.

Behind every man now alive stand thirty ghosts, for that is the ratio by which the dead outnumber the living.

When you finally understand the universe, it will not only be stranger than you imagine, it will be stranger than you can imagine.

The exploration of the planets is now closer to us in time than the exploration of Africa by Stanley and Livingstone.

I have great faith in optimism as a guiding principle, if only because it offers us the opportunity of creating a self-fulfilling prophecy.