Human knowledge is produced by intellectual combat that exposes weak premises and faulty conclusions to withering challenge. We are often improved more by our ideological enemies than by our friends, because our enemies push us hardest. In that spirit, I welcome the publication of Paul Krugman’s “The Conscience of a Liberal” (W.W. Norton, 352 pages, $25.95). The book espouses a world-view that is in many ways diametrically opposed to my own, but the process of intellectually disagreeing with Mr. Krugman fired my own passion for liberty more than the rhetoric of any current GOP presidential candidate does.

Mr. Krugman has written a sweeping political history of the past 135 years from a stridently liberal Democratic viewpoint. In Krugman’s worldview, noble Democratic progressives have long battled a conspiracy of Republican knaves who are themselves the pawns of selfish plutocrats.

He advances his viewpoint not by misstating facts but by omitting those parts of the past that make history messier. He expresses outrage that Democrat Samuel Tilden “essentially had the electoral vote stolen” in 1876, but does not mention that Tilden’s Southern victories were achieved through the violent suppression of black votes by Democratic henchmen and the Ku Klux Klan. He derides Barry Goldwater for his long-standing support of Joseph McCarthy, but does not seem disturbed that John F. Kennedy also chose not to censure “Tailgunner Joe.” We read a great deal about Nixon’s Southern strategy and implicit Republican appeals to racism after 1964, but little about the explicit Democratic strategy of race hatred that was the norm among many of leading Democratic legislators such as Theodore Bilbo. But while Mr. Krugman’s prose is one-sided, his two major themes are correct. His first theme is that the Democratic party has long battled inequality. His second theme is that Republicans have done a lot of dubious things in their quest for political dominance.

The historical connection between the Democratic party and equality goes back to its founding heroes: Thomas Jefferson and Andrew Jackson. During the 20th century, Democrats have favored redistribution and social policies that would make America more equal. There was less inequality during the great era of Democratic political dominance, from 1933 to 1968, than either before or after that epoch. Mr. Krugman is also right that equality — if it comes at no cost — is surely a good thing. But the price of equality can be the loss of economic productivity and, more importantly, of freedom.

Reading “The Conscience of a Liberal” left me hungry for a rival book retelling history from an equally strident Republican perspective. Such a book would have two great themes that would be just as true as Mr. Krugman’s central tenets. The first theme would be that the Republican party has long fought for freedom. The second theme would be that Democrats have done a lot of dubious things in their quest for power.

Such a book might start with the battle against slavery and remind us that while “not every Democrat was a rebel, every rebel was a Democrat.” It would recount the century-long battle of leading Democrats to support Jim Crow and note that the majority of votes for the Civil Rights Act of 1957 were cast by Republicans. Such a history of the past 140 years would emphasize the growth of prosperity at home and freedom abroad under Republican leadership that put liberty first. American success could be contrasted with the gruesome results that often came from entrusting political power to those who loved equality most in Europe. Neither the New Deal nor the Great Society were unmixed blessings. The excesses of both expansions of the reach of government still serve as warnings about the dangers of too much intervention.

It is important that America continue to debate the relative merits of liberty and equality, and I am glad that Mr. Krugman is out there fighting for equality. I only wish that the friends of freedom were still making the case for liberty with as much vigor.

Mr. Krugman also does well by reminding us of the flaws of Republicans such as Richard Nixon who used dirty tricks and misinformation for political gain. Certainly, the last Republican legislature passed far too many earmarked projects for their pet supporters. Of course, a fuller history would emphasize that the GOP has no monopoly on vice. Corruption has been part of the Democratic party since Aaron Burr turned Tammany Hall into a political machine and used his political clout to get banking privileges for the Manhattan Water Company. Robert Caro’s biography of Lyndon Johnson contains hundreds of pages indicting one of the last century’s most important Democrats. And it would be hard to argue that Jimmy Carter was a model of presidential competence.

Men are sinners and politics ain’t beanbag. The inherent imperfection of our leaders has led conservatives, for centuries, to wisely limit the power of the state. Moreover, the inherent flaws in all of us counsel a profound political and social humility that should take the form of civility toward even our most bitter opponents.

I doubt that this was Mr. Krugman’s intended message, but it strikes me as the natural implication of the many misdeeds on both sides of the political aisle. His book is not a pleasant read for those who don’t share his views, but Republicans have much to learn from their Democratic opponents, just as Democrats have much to learn from the party of Lincoln.

Mr. Glaeser is the Glimp professor of economics at Harvard, director of the Taubman Center for State and Local Government, and a senior fellow at the Manhattan Institute.