Freedom of opinion can only exist when the government thinks itself secure.

All exact science is dominated by the idea of approximation.

Men fear thought as they fear nothing else on earth — more than ruin — more even than death…. Thought is subversive and revolutionary, destructive and terrible, thought is merciless to privilege, established institutions, and comfortable habit. Thought looks into the pit of hell and is not afraid. Thought is

Change is scientific, progress is ethical; change is indubitable, whereas progress is a matter of controversy.

To be able to fill leisure intelligently is the last product of civilization, and at present very few people have reached this level.

Many people would sooner die than think; In fact, they do so.

I would never die for my beliefs because I might be wrong.