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 must be the truth.

On Euler's identity, e^{i \pi} + 1 = 0. \,\! as quoted in notes by W. E. Byerly, published in Benjamin Peirce, 1809-1880: Biographical Sketch and Bibliography (1925) by R. C. Archibald; also in Mathematics and the Imagination (1940) by Edward Kasner and James Newman.