One must judge men not by their opinions, but by what their opinions have made of them.

To err is human also in so far as animals seldom or never err, or at least only the cleverest of them do so.

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.

People who have read a good deal rarely make great discoveries. I do not say this in excuse of laziness, but because invention presupposes an extensive independent contemplation of things.

The most entertaining surface on earth is the human face.

I am grateful that I am not as judgmental as all those censorious, self-righteous people around me. In each of us there is a little of all of us.

Everyone is a genius at least once a year.