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.*’