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