The deepest definition of youth is life as yet untouched by tragedy.

Vigorous societies harbor a certain extravagance of objectives, so that men wander beyond the safe provision of personal gratifications.

Art flourishes where there is a sense of adventure.

No science can be more secure than the unconscious metaphysics which tacitly it presupposes.

But you can catch yourself entertaining habitually certain ideas and setting others aside; and that, I think, is where our personal destinies are largely decided.

Every really new idea looks crazy at first.

Civilization advances by extending the number of important operations which we can perform without thinking of them.