Many theories of the ancient world seem terribly childish today, a hodge-podge of fables and false comparisons.But our theories will seem childish five-hundred years from now.Every theory is based on some analogy, and sooner or later the theory fails because the analogy turns out to be false. A theory in its day helps to solve the problems of the day.

Sex was invented as a biological instrument by (say) the green algae. But as an instrument in the ascent of man which is basic to his cultural evolution, it was invented by man himself.

A genius is a man who has two great ideas.

Nations in their great ages have not been great in art or science, but in art and science.

Revolutions are not made by fate but by men.

Man is a singular creature. He has a set of gifts which make him unique among the animals, so that unlike them, he is not a figure in the landscape, he is the shaper of the landscape.

The most remarkable discovery ever made by scientists is science itself. The discovery must be compared in importance with the invention of cave-painting and of writing.