Pessimism and optimism are the two ends of a spectrum that spans one of the many dimensions of the individual human personality. We observe that any given human being has a disposition that does not stray far from some particular point on that spectrum; that this disposition appears early in life, and may very well be congenital; and that, if not completely immutable, it is very persistent. The pre-modern physicians placed this disposition in their Four Humors theory: the sanguine humor inclining to optimism, the melancholic to pessimism.

Both tendencies have natural advantages to the individual. The principal use of pessimism is to restrain the follies that arise from unbridled optimism, and vice versa. The chronic gambler who gives over all his foresight to an expectation of winning would benefit from a dash of pessimism; the Eeyore who believes in the futility of all endeavor is not likely to accomplish much unless someone drops an...