(source) 
Joseph Liouville
(24 Mar 1809  8 Sep 1882)

Quotes by others about Joseph Liouville (1)
Once when lecturing to a class he [Lord Kelvin] used the word mathematician, and then interrupting himself asked his class: Do you know what a mathematician is? Stepping to the blackboard he wrote upon it: [an integral expression equal to the square root of pi]
Then putting his finger on what he had written, he turned to his class and said: A mathematician is one to whom that is as obvious as that twice two makes four is to you. Liouville was a mathematician.
Then putting his finger on what he had written, he turned to his class and said: A mathematician is one to whom that is as obvious as that twice two makes four is to you. Liouville was a mathematician.
See also:
 24 Mar  short biography, births, deaths and events on date of Liouville's birth.
 Joseph Liouville 18091882: Master of Pure and Applied Mathematics, by Jesper Lόtzen.  book suggestion.