From The Life of Lord Kelvin by Silvanus Phillips Thompson (American Mathematical
Society, 1976), page 1139, footnote 1:

Once when lecturing in class he [the 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:—

\int_{-\infty}^\infty e^{-x^2}\,dx=\sqrt{\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. [...]“

Prof Coveney made a reference to this passage in the lecture on Friday. That is the reason why I am going to waste more time in libraries to understand the Gaussian integral, or I have to put Lord Kelvin on the list of famous scientists I’d like to [author's discretion].