Voici une courte anecdote de Lord Kelvin, professeur.

Once when lecturing he 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 :

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 he resumed his lecture.

S. P. Thompson, The Life of William Thomson, Baron Kelvin of Largs, p. 1139