There are three classes of people: those who see, those who see when they are shown, those who do not see.

A wave is never found alone, but is mingled with as many other waves as there are uneven places in the object where the said wave is produced.

Oh! Speculators on things, boast not of knowing the things that nature ordinarily brings about; but rejoice if you know the end of those things which you yourself device.

People reveal themselves completely only when they are thrown out of the customary conditions of their life.

The painter will produce pictures of little merit if he takes the works of others as his standard.

He who does not understand the supreme certainty of mathematics is wallowing in confusion.

He who wishes to be rich in a day will be hanged in a year.