A fanatic is a man who, when he's lost sight of his purpose, redoubles his effort.

I wrote the first book, Harvest of Stars, and as I was writing it, I saw that certain implications had barely been touched on... It's perfectly obvious that two completely revolutionary things are going on, with cybernetics, and biological science.

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

Happier are all men than the dwellers in Faerie – or the gods, for that matter…Better a life like a falling star, bright across the dark, than a deathlessness that can see naught above or beyond itself…the day draws nigh when Faerie shall fade, the Erlking himself shrink to a woodland sprite and then to nothing, and the gods go under. And the worst of it is, I cannot believe it wrong that the immortals will not live forever.

We live with our archetypes, but can we live in them?

My knowledge of the human psyche is as yet imperfect. Certain areas won't yield to computation.

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.