Never undertake anything for which you wouldn't have the courage to ask the blessings of heaven.

The wisdom of providence is as much revealed in the rarity of genius, as in the circumstance that not everyone is deaf or blind.

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

What we have to discover for ourselves leaves behind in our mind a pathway that can be used on another occasion.

If another Messiah was born he could hardly do so much good as the printing-press.

As soon as you know a man to be blind, you imagine that you can see it from his back.

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