After years of finding mathematics easy, I finally reached integral calculus and came up against a barrier. I realized that this was as far as I could go, and to this day I have never successfully gone beyond it in any but the most superficial way.

Old men tend to forget what thought was like in their youth; they forget the quickness of the mental jump, the daring of the youthful intuition, the agility of the fresh insight. They become accustomed to the more plodding varieties of reason, and because this is more than made up by the accumulation of experience, old men think themselves wiser than the young.

Individual science fiction stories may seem as trivial as ever to the blinder critics and philosophers of today - but the core of science fiction, its essence has become crucial to our salvation if we are to be saved at all.

Working ten hour days allows you to fall behind twice as fast as you could working five hour days.

No sensible decision can be made any longer without taking into account not only the world as it is, but the world as it will be.

A fire-eater must eat fire even if he has to kindle it himself.

Despite all that education and experience can do, I retain a certain level of unsophistication that I cannot eradicate and that my friends find amusing. In fact, I think I sometimes detect conspiratorial plottings among my friends to protect me against my own lack of sophistication. I don't mind. I suspect that I am never quite as unsophisticated as they think I am, but I don't mind.