Once when lecturing in class Lord Kelvin used the word…

Once when lecturing in class Lord Kelvin used
the word ‘mathematician’ and then interrupting himself asked his class:
Do you know what a mathematician is?’

Stepping to his blackboard he
wrote upon it the above equation.

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.