Knowledge is two-fold, and consists not only in an affirmation of what is true, but in the negation of that which is false.

True friendship is like sound health; the value of it is seldom known until it is lost.

We owe almost all our knowledge not to those who have agreed, but to those who have differed

That writer does the most who gives his reader the most knowledge and takes from him the least time

True contentment depends not upon what we have; a tub was large enough for Diogenes, but a world was too little for Alexander.

Physical courage, which despises all danger, will make a man brave in one way; and moral courage, which despises all opinion, will make a man brave in another.