No trilogy should have more than four books.

The numbers of distinct human societies or nations, when our race is twice its present age, may be far greater than the total number of all the men who have ever lived up to the present time.

I'm sure the universe is full of intelligent life. It's just been too intelligent to come here.

I sometimes think that the universe is a machine designed for the perpetual astonishment of astronomers.

I want to be remembered most as a writer - one who entertained readers, and, hopefully, stretched their imagination as well.

The difference between machines and human beings is that human beings can be reproduced by unskilled labour.

The time was fast approaching when Earth, like all mothers, must say farewell to her children.