Benjamin Peirce (1809 - 1880)

[after proving Euler's formula e πi = -1 in a lecture]
Gentlemen, that is surely true, it is absolutely paradoxical; we cannot understand it, and we don't know what it means. But we have proved it, and therefore we know it is the truth.

Quoted in E Kasner and J Newman Mathematicas and the Imagination (New York 1940)