Why is it that democracy is now the sole legitimate form of government throughout the world? Democracy had been deplored, even despised, from Plato to “The Federalist” — more perhaps for inconstancy and unreliability than viciousness. Yet now all (or almost all) regimes, the worst as much as the best, feel obliged to declare themse lves to be democracies.

John Dunn, the distinguished political theorist at Cambridge University, answers this question with learning and sophistication in “Democracy: A History” (Atlantic Monthly Press, 248 pages, $24). He shares the present-day view that there is no legitimate alternative to democracy, but he does not feel enthusiastic about democracy. It is not that he wants another choice of government, but as a chastened left-winger he clearly wishes that democratic voters today were not so attached to the “order of egoism,” his rather disdainful name for capitalism. (The phrase is from Philippe Buonarroti, a French socialist theorist of Italian origin active in the French Revolution.)

The word “democracy” comes from the ancient Greeks, and so too does the grandest celebration of democracy in the Funeral Speech of Pericles (430 B.C.) preserved or composed for us by Thucydides. Pericles asserts that the Athenians do not imitate others but themselves offer a model, and what is best about them is not their beautiful buildings but their beautiful regime. “For we are lovers of beauty with thrift, and lovers of wisdom without softness” — a reproach to today’s aesthetes and intellectuals, Mr. Dunn notes. Pericles goes on to say that Athenians regard a man who takes no part in public affairs not as one who minds his own business but as good for nothing. No order of egoism here!

Among the Greeks democracy, the rule of the demos, or the many, had a rival in oligarchy, the rule of the few. In Thucydides’s “History of the Peloponnesian War” democratic Athens stood against oligarchic Sparta, and the reader has a difficult time deciding which is better. Each regime has a cluster of virtues and accompanying vices, for example the daring of Athens together with its volatility, and the moderation of Sparta combined with its inhumanity to enemies. Despite what Pericles says, there is no clear superiority for democracy in Thucydides’s “History.”

Plato and Aristotle generally rank oligarchy over democracy. Plato did not “loathe” democracy, as Mr. Dunn claims. Plato believes that democracy with its laxity is more tolerant of philosophy than is oligarchy, which is harsher, less tolerant of vice but also of criticism. Plato’s penetrating critique of democracy in “The Republic” is relevant today. He writes that the tendency for democratic diversity to descend into conformity as a consequence of the requirement of every democrat to become “diverse,” just like everyone else. Aristotle says that man is by nature a political animal, but that there is no single regime by nature; there are several. Human beings are both equal and unequal to each other, and democracy, which appeals to our equality, is not always preferable to oligarchy, which is based on our inequalities. Aristotle appreciates the “good naturedness” of the multitude. He thinks that its various qualities can sometimes add up to more quality than can be had from one person or a few, and that the many judge better than the experts in the same way that the customer knows what he wants more than the cook. But on the whole, the best is one person or a few, and they are more often impeded than welcomed by democracy.

Thus, to explain why democracy has become the only legitimate government, we need to know why oligarchy has dropped out of contention, and why the rivalry between the two, epitomized in Athens vs. Sparta, is not found in modern politics. What is special about modern politics?

The answer to this, Mr. Dunn indicates, is representative government, invented by Thomas Hobbes in the 17th century. Hobbes (followed by Spinoza and Locke) argued that government is properly instituted by contract among individuals in the state of nature who authorize a sovereign over themselves to represent them. The crucial new concept in this theory is the state of nature that precedes all society, is therefore insecure and full of violence, and is further characterized by the rough equality of all those who experience or imagine it. Men are equal in the state of nature, according to Hobbes, for the very realistic reason that no one can be sure of always getting the better of (i.e. killing) anyone or everyone else. Here, democracy is installed as the foundation of all government.

Democracy in the basement, however, is not necessarily democracy in the house above. A representative government may not hold elections (such as the hereditary monarchy that Hobbes wanted), and if it does, the elections may not be democratic, with universal suffrage. Representative democracy came later, in the American and French Revolutions, and of course women’s suffrage was still later.Yet the nature of democracy is changed when it becomes representative. No longer does the people meet in a body to deliberate on and decide the most important questions of government, but rather the task of actual governing is delegated to a few elected (and non-elected) representatives. In this delegation, as we are often reminded, lies a hidden oligarchy dressed up as democratic. If every representative government is fundamentally democratic, every modern democracy is in practice oligarchic. The ancients knew of a mixed regime composed of democracy and oligarchy, but this is a mixed regime of a new kind in which no one speaks of oligarchy in its principle though everyone find it useful in practice.

Without too much groveling apology, Mr. Dunn manfully swallows the extension of democracy to include women, but his main interest is the French Revolution. Not that he forgets America, he prefers a European view of the history of democracy, one that features the Abbé Sieyes, Buonarroti, and Gracchus Babeuf. He does not accept Tocqueville’s assertion that democracy is in — belongs to — America. For this reason he underestimates the value of ambition in democracy.

Mr. Dunn’s analysis turns sober and profound when he presents the opposition between democracy as a form of government and as a way of life. As a way of life (best shown by Tocqueville) democracy is not satisfied with existing equality but tries relentlessly to equalize everything. In this sense democracy is open-ended, or points toward a goal of no power or government at all. As a form of government, however, democracy tries to contain this infinite democratization. One mode of containment is to accept, as democratic electorates even in Europe have done recently, the “order of egoism” that socialists like Babeuf and Buonarroti hate. Mr. Dunn worries that “malign consequences” will follow from the separation of the democratic ethics of equalization from the democratic reality of surrender to egoism.

One may agree that it is harmful to the souls of intellectuals, as Mr. Dunn worries, when they cherish their own moral purity as against the evils of reality. But a remedy can be found in the notion of “self-interest well understood” that Tocqueville attributes to Americans. “Egoism” in America, both in business and in government, is more about ambition than greed for money. The socialists, obsessed with money as they believed their capitalist enemies to be, always missed the point. Money is a sign of success, and success is the satisfaction of one’s ambition. It is not necessary to go against egoism or self-interest in order to be public spirited. Ambition was in Aristotle’s list of virtues and it is featured in James Madison’s famous discussion of the separation of powers in “The Federalist.” The history of democracy is the democratization, not the elimination, of oligarchic ambition.

Mr. Mansfield is a professor of government at Harvard University and the author of “Manliness,” available from Yale University Press.