A handy short definition of almost all science fiction might read: realistic speculation about possible future events, based solidly on adequate knowledge of the real world, past and present, and on a thorough understanding of the nature and significance of the scientific method.

When the citizens of a nation will no longer volunteer to defend it, then it is probably not worth saving. No nation has the right to survive with conscript troops, and in the long run, no nation ever has.

By the data to date, there is only one animal in the Galaxy dangerous to man -- man himself. So he must supply his own indispensable competition. He has no enemy to help him.

Death isn't funny." "Then why are there so many jokes about death? Jill, with us — us humans — death is so sad that we must laugh at it.

Animals can be driven crazy by placing too many in too small a pen. Homo sapiens is the only animal that voluntarily does this to himself.

Evolution is a process that never stops. Baboons who fail to exhibit moral behavior do not survive; they wind up as meat for leopards.

Everything is theoretically impossible, until it is done.