You may reasonably expect a man to walk a tightrope safely for ten minutes; it would be unreasonable to do so without accident for two hundred years.

What Galileo and Newton were to the seventeenth century, Darwin was to the nineteenth.

Each act of cruelty is eternally a part of the universe; nothing that happens later can make that act good rather than bad, or can confer perfection on the whole of which it is a part.

In science the successors stand upon the shoulders of their predecessors; where one man of supreme genius has invented a method, a thousand lesser men can apply it. ... In art nothing worth doing can be done without genius; in science even a very moderate capacity can contribute to a supreme achievement.

Our individual life is brief, and perhaps the whole life of mankind will be brief if measured in astronomical scale

Not to be absolutely certain is, I think, one of the essential things in rationality.

The axiomatic method has many advantages over honest work.