On November 6th, more than a hundred million Americans will go to the polls and choose between two Presidential candidates. If you live a full life span, you will get to vote for President about fifteen times before you die, and for members of Congress somewhat more often. In the opinion of almost all of us, this entitles us to say that we live in a democracy—a word that comes from the ancient Greeks, for whom a democracy was a government in which the people, the demos, ruled. If you are a member of the American demos, however, and ask yourself in what sense you really have a hand in ruling the country, the answer is not obvious. You can vote for a President, but you will almost certainly never get to be President yourself. Unless you are one of the few people to serve in elected or appointed office, you will never personally decide whether America should go to war or make peace; you will never draft a law or vote on its passage; you will never decide a constitutional case. You will never decide on zoning issues or tax rates, building projects or environmental policy. Perhaps, once or twice in a lifetime, you will serve on a jury and help decide whether to send a person to prison—the only time you will experience what it means to have power over your fellow-citizens.

What would American government look like if it began from the principle that people in a democracy should exercise the levers of power individually and collectively? The fairest way to make sure that everyone had an equal chance to rule would be a lottery. Imagine, then, that on November 6th, instead of going to the polls, you opened your e-mail and found a message informing you that, for the next term of office, you would be a senator, or a mayor, or a city councilman. Of course, as Alan Ryan notes in “On Politics” (Norton), his magisterial new two-volume history of political philosophy, there are so many people in the United States that it would be hard to give everyone a chance to serve. If, in the next four years, every adult in New York City were to be mayor for an equal term, that term would be just twenty seconds long.

To avoid such absurdities, our jurisdictions would have to be much smaller: a city, or even a country, would have to contain no more than about twenty thousand citizens, to insure that every adult had a chance to hold power for a meaningful amount of time, at least once. Such small polities would be bound to come into frequent conflict, and much of our time would be spent fighting or preparing to fight; military service would be a prerequisite for political eligibility. Naturally, spending so much time fighting and thinking about politics would mean that we had much less time for making a living. So we would have to designate a class of people to do the work for us—ideally, robots, but, failing that, slaves, ones that we either bought or took prisoner in war. And if we were to exercise real power as rulers, no part of the government should be off-limits to any of us: we should be able to judge criminal cases, make laws, and choose generals, as the situation demanded.

In pursuit of democracy, then, we would end up with a society with immutable social hierarchies, without checks and balances or a separation of powers—a slave state in which the only life that mattered would be public life, and individual rights would be irrelevant. In other words, we would end up with an ancient Greek polis, a city much like Athens in its prime, in the fifth century B.C. For thousands of years, the West has admired Athens and paid tribute to it as the birthplace of democracy. Yet most of us would find such a city not only undesirable but unfree and unjust. In achieving true democracy, we would come up with a form of government that violates what we think of as liberty.

This paradox was never more clearly stated than by the French politician Benjamin Constant, in his 1819 lecture “The Liberty of the Ancients Compared with That of the Moderns.” Speaking to an audience that had lived through the Revolution, the Terror, the Napoleonic Empire, and the restoration of the monarchy, Constant asked what sort of freedom modern people really valued. He argued that in the modern age liberty meant, primarily, freedom from coercion and interference. People want to be free from arbitrary imprisonment, free to speak their minds, free to choose their professions and associates. To the ancient Greeks and Romans, on the other hand, freedom meant something much more positive: freedom to participate in government decisions, to make laws and declare war and judge criminals. Yet this ancient liberty came at a price that we would find prohibitively high—“the complete subjection of the individual to the authority of the group.” A Spartan lyre player, Constant noted, once got into legal trouble for adding a new string to his instrument; how much less would a Spartan expect to have the freedom to marry for love, to set up in business, to stay home when others went to war?

Constant wrote, “The aim of the ancients was to share power among the citizens of a single country; that’s what they called ‘liberty.’ The aim of the moderns is to be secure in their private benefits; and ‘liberty’ is their name for the guarantees accorded by institutions to these benefits.” The error of the French Revolution, he argued, was that it expected modern individualists to build a Republic of Virtue as austere and demanding and total in its claims as any ancient polis. But life had changed too much for ancient institutions to serve modern men, who “should never be asked to make sacrifices in order to establish political liberty.” True happiness, he concluded, requires combining ancient and modern liberty, the immunities of liberalism with the commitments of democracy.

A book of the scope of “On Politics” can’t be reduced to a single theme. In more than a thousand pages, Alan Ryan, a longtime Oxford professor who now teaches at Princeton, undertakes to introduce the reader to most of the major political thinkers in Western history, from Thucydides and Plato to John Dewey and John Rawls. “A colleague,” Ryan writes, “once described political theorists as people who were obsessed with two dozen books,” and at the heart of “On Politics” are accounts of the major works of Western political thought—by Aristotle, Cicero, Augustine, Machiavelli, Hobbes, Locke, Rousseau, Hegel, Mill, and Marx, among others. These share the stage with lesser or less explicitly political figures, including Dante (in his capacity as a theorist of universal monarchy), the English republican Algernon Sidney, and the Third World liberationist Frantz Fanon. As Ryan approaches the present, he also allows a number of political scientists and sociologists to join the pure philosophers, from Max Weber to David Riesman.

In general, Ryan’s itinerary is what you would expect from an Anglo-American professor of political theory: ample attention is paid to the drafting of the Constitution, for instance, while thinkers from the Continent are in greater danger of being omitted; there is little to nothing on Vico, Spinoza, Nietzsche, or Sartre. Ryan’s own intellectual focus is the liberal tradition—earlier this year, he published a collection of essays under the title “The Making of Modern Liberalism,” and he has written studies of sages like Dewey and Mill. The tensions of modern liberal democratic societies are the intellectual motor of the book. Thus the second volume, which covers the past three hundred and fifty years, is a good deal longer than the first, which covers the previous two thousand years. It is also more lively and intellectually engaging, as Ryan enters into the range of problems that continue to preoccupy us in the twenty-first century. Although Ryan, oddly, does not devote a section to Constant, it is clear that his lecture, which Ryan calls “a liberal sacred text,” is a cornerstone of the book.