If we knew exactly what to expect throughout the Solar System, we would have no reason to explore it.

He had seen too much of the cosmos to have any great faith in man's ability to understand it.

I have yet to see any problem, however complicated, which, when you looked at it in the right way, did not become still more complicated.

You know what they say about bold spacemen never becoming old spacemen.

Why do people in this age think their own impoverished lives must be the norm of the universe?

What five books would I like to be remembered for? Well... Tau Zero, I like that one especially. It was somewhat of a tour de force, and I think it got across what I was trying for.

Will none wipe the sneer off the face of the cosmos?

At each stage...entirely new laws, concepts and generalizations are necessary, requiring inspiration and creativity to just as great a degree as in the previous one.

The single definition of government I've ever seen that makes sense is that it's the organization which claims the right to kill people who won't do what it wants.

Two lives met across death and centuries. To ask what it meant is meaningless. There is no destiny. But sometimes there is bravery

In my considered opinion, the profit to be made by permanent settlement in space is nothing less than the survival of industrial civilization, and therefore the survival of nearly the entire human race, along with such amenities as peace, freedom, enough to eat, and the chance to reach a high age in good health.

Better a life like a falling star, brief bright across the dark, than the long, long waiting of the immortals, loveless and cheerlessly wise.

Give fear no hold on you. Keep sinews loose and senses open, ready at every instant to flow with the rush of action.