“The hours of folly are measured by the clock, but of wisdom no clock can measure.”

“Less than fifteen percent of the people do any original thinking on any subject . . . The greatest torture in the world for most people is to think”

“It is well for people who think to change their minds occasionally in order to keep them clean. For those who do not think, it is best to at least rearrange their prejudices once in a while”

“If we would guide by the light of reason we must let our minds be bold”

“The man who never alters his opinions is like standing water, and breeds reptiles of the mind”

“Not to unlearn what you have learned is the most necessary kind of learning.”