In this winter of their discontents, nostalgia for Ronald Reagan has become for many conservatives a substitute for thinking. This mental paralysis -- gratitude decaying into idolatry -- is sterile: Neither the man nor his moment will recur. Conservatives should face the fact that Reaganism cannot define conservatism.

That is one lesson of John Patrick Diggins's new book, " Ronald Reagan: Fate, Freedom, and the Making of History." Diggins, a historian at the City University of New York, treats Reagan respectfully as an important subject in American intellectual history. The 1980s, he says, thoroughly joined politics to political theory. But he notes that Reagan's theory was radically unlike that of Edmund Burke, the founder of modern conservatism, and very like that of Burke's nemesis, Thomas Paine. Burke believed that the past is prescriptive because tradition is a repository of moral wisdom. Reagan frequently quoted Paine's preposterous cry that "we have it in our power to begin the world over again."

Diggins's thesis is that the 1980s were America's "Emersonian moment" because Reagan, a "political romantic" from the Midwest and West, echoed New England's Ralph Waldo Emerson. "Emerson was right," Reagan said several times of the man who wrote, "No law can be sacred to me but that of my nature." Hence Reagan's unique, and perhaps oxymoronic, doctrine -- conservatism without anxieties. Reagan's preternatural serenity derived from his conception of the supernatural.

Diggins says Reagan imbibed his mother's form of Christianity, a strand of 19th-century Unitarianism from which Reagan took a foundational belief that he expressed in a 1951 letter: "God couldn't create evil so the desires he planted in us are good." This logic -- God is good, therefore so are God-given desires -- leads to the Emersonian faith that we please God by pleasing ourselves. Therefore there is no need for the people to discipline their desires. So, no leader needs to suggest that the public has shortcomings and should engage in critical self-examination.

Diggins thinks that Reagan's religion "enables us to forget religion" because it banishes the idea of "a God of judgment and punishment." Reagan's popularity was largely the result of "his blaming government for problems that are inherent in democracy itself." To Reagan, the idea of problems inherent in democracy was unintelligible because it implied that there were inherent problems with the demos -- the people. There was nothing -- nothing-- in Reagan's thinking akin to Lincoln's melancholy fatalism, his belief (see his Second Inaugural) that the failings of the people on both sides of the Civil War were the reasons why "the war came."

As Diggins says, Reagan's "theory of government has little reference to the principles of the American founding." To the Founders, and especially to the wisest of them, James Madison, government's principal function is to resist, modulate and even frustrate the public's unruly passions, which arise from desires.

"The true conservatives, the founders," Diggins rightly says, constructed a government full of blocking mechanisms -- separations of powers, a bicameral legislature, and other checks and balances -- in order "to check the demands of the people." Madison's Constitution responds to the problem of human nature. "Reagan," says Diggins, "let human nature off the hook."

"An unmentionable irony," writes Diggins, is that big-government conservatism is an inevitable result of Reaganism. "Under Reagan, Americans could live off government and hate it at the same time. Americans blamed government for their dependence upon it." Unless people have a bad conscience about demanding big government -- a dispenser of unending entitlements -- they will get ever larger government. But how can people have a bad conscience after being told (in Reagan's First Inaugural) that they are all heroes? And after being assured that all their desires, which inevitably include desires for government-supplied entitlements, are good?

Similarly, Reagan said that the people never start wars, only governments do. But the Balkans reached a bloody boil because of the absence of effective government. Which describes Iraq today.

Because of Reagan's role in the dissolution of the Soviet Union, Diggins ranks him among the "three great liberators in American history" -- the others being Lincoln and Franklin Roosevelt -- and among America's three or four greatest presidents. But, says Diggins, an Emersonian president who tells us our desires are necessarily good leaves much to be desired.

If the defining doctrine of the Republican Party is limited government, the party must move up from nostalgia and leaven its reverence for Reagan with respect for Madison. As Diggins says, Reaganism tells people comforting and flattering things that they want to hear; the Madisonian persuasion tells them sobering truths that they need to know.

georgewill@washpost.com