A vacuum of ideas affects people differently than a vacuum of air, otherwise readers of books would be constantly collapsing.

Doubt everything at least once, even the sentence "Two times two is four."

Great men too make mistakes, and many among them do it so often that one is almost tempted to call them little men.

A person reveals his character by nothing so clearly as the joke he resents.

Love is blind, but marriage restores its sight.

A good method of discovery is to imagine certain members of a system removed and then see how what is left would behave: for example, where would we be if iron were absent from the world: this is an old example.

It often takes more courage to change one's opinion than to stick to it.