American conservatism, Mr. Will reflects, is about the conservation of the wisdom of the nation’s founding; otherwise “it is nothing of much lasting significance.” He seeks to show “the continuing pertinence of the Founding principles” and to trace “many of our myriad discontents to departures from those principles.”

To simplify: Mr. Will understands the previous century of American political debate as a contest between conservatism, rooted in the Founders’ conception of natural rights and a fixed human nature, and progressivism, which either posits a malleable human nature or ignores man’s limits altogether. The Founders drew on the ancients, who asked how a polity might encourage man, with all his limitations and self-interest, to achieve the highest forms of behavior of which he is capable, and the early moderns, especially Locke and Hobbes, who asked how a polity might stop man from descending into tyranny and self-annihilation. Both ancients and early moderns believed, as conservatives have believed since, that human nature is unchangeable.

Progressives, by sharp contrast—Herbert Croly, John Dewey and others in the intellectual sphere; Teddy Roosevelt and Woodrow Wilson in the political sphere—relied mainly on Hegel’s teleological historicism and believed that human nature could be molded and improved in ways that held out limitless possibility. The core doctrine of modern progressivism, Mr. Will writes, “is that human nature has no constancy, that it is merely an unstable imprint of the fluctuating social atmosphere.” […]