“Man is many things, but he is not rational.”

Source: Tunnel in the Sky (1955), Chapter 2, “The Fifth Way” (p. 42)

Barry Mazur,
Context: Sometimes the mathematical anti-Platonist believes that headway is made by showing Platonism to be unsupportable by rational means, and that it is an incoherent position to take when formulated in a propositional vocabulary. It is easy enough to throw together propositional sentences. But it is a good deal more difficult to capture a Platonic disposition in a propositional formulation that is a full and honest expression of some flesh-and-blood mathematician’s view of things. There is, of course, no harm in trying—and maybe its a good exercise. But even if we cleverly came up with a proposition that is up to the task of expressing Platonism formally, the mere fact that the proposition cannot be demonstrated to be true won’t necessarily make it vanish. There are many things—some true, some false— unsupportable by rational means.

Source: Hard Day's Knight

19 December 1749
Letters to His Son on the Art of Becoming a Man of the World and a Gentleman (1774)
Context: We must not suppose that, because a man is a rational animal, he will, therefore, always act rationally; or, because he has such or such a predominant passion, that he will act invariably and consequentially in pursuit of it. No, we are complicated machines; and though we have one main spring that gives motion to the whole, we have an infinity of little wheels, which, in their turns, retard, precipitate, and sometime stop that motion.

Pudd'nhead Wilson's New Calendar, Ch. XXI
Following the Equator (1897)
Variant: Man will do many things to get himself loved, he will do all things to get himself envied.

Book 6, chapter 12.
Books, Coningsby (1844), Henrietta Temple (1837)

Source: The War Hound and the World's Pain (1981), Chapter 14 (p. 142)

Source: Beautiful Ruins

Vol. 2, Ch. 24 "Oracular Philosophy and the Revolt against Reason"
The Open Society and Its Enemies (1945)