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 his 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."
from: S. P. Thompson, The Life of William Thomson, Baron Kelvin of Largs, as seen on "Lord Kelvin Quotations" website, http://zapatopi.net/kelvin/quotes/