In the mid-1980s, many Americans began to worry that religious pressures were slowly distorting American institutions. The growing political influence of the religious right made many fear values of mutual respect and equality might be forgotten in the country's zeal for religious solidarity. In addressing this sense of alarm, the political philosopher John Rawls (1921-2002) invoked the history of the Weimar Republic, saying that things go from bad to worse when intellectuals are unwilling to address the urgent issues of their time. In "Political Liberalism" (1993), he resolved to do just that.

Rawls was already famous for "A Theory of Justice" (1971), one of the philosophical classics of the 20th century, which set out basic principles for a just society. Such a society, he argued, would commit itself to the greatest possible liberty that is compatible with a like liberty for all, and would also permit economic inequality only when that raised the level of the least well-off. Rawls never withdrew or in any major way altered the principles for which he had argued in that work. He also continued to endorse its most innovative philosophical device, the "Original Position," according to which political principles are imagined as selected by rational agents who are deprived of all information about their place in society (wealth, class, race, gender, religion, etc.) that would bias the design of principles in favor of their own situation. Rawls remained unswervingly committed to impartiality and equal respect as core political values, and to the selection procedure based upon these values.

In light of the growing religious presence in American public life, however, Rawls felt the need to augment and revise his great work, attempting to show people with deep religious convictions that they had good reason to accept principles that guaranteed liberty and fairness to all. The fruits of Rawls's decade of reflection on this problem became "Political Liberalism," a book less famous today than "A Theory of Justice," but even more urgently relevant for our own time, as we struggle, once again, with problems of religious solidarity and equal respect.

Rawls's "Political Liberalism" asks an urgent question: Can liberal constitutional democracy, built on values of mutual respect and reciprocity, be stable, or even survive, in a world of religious and secular pluralism? Or, to use his words, "[H]ow is it possible for there to exist over time a just and stable society of free and equal citizens who remain profoundly divided by reasonable religious, philosophical, and moral doctrines?"

Rawls puts the question in this way  "how is it possible"  because he is not convinced that such a thing is possible. Indeed, the introduction he added to the paperback edition of 1996 expresses real anguish on that score. The events of the twentieth century, he says there, raise real doubts about the fate of justice in this world. But if the question cannot be answered in the affirmative, and people are largely amoral and self-centered, then "one might ask with Kant whether it is worthwhile for human beings to live on the earth." We must therefore, he says, begin "with the assumption that a reasonably just political society is possible," and with the related assumption that human beings have enough of a moral nature that they can be moved by considerations of fairness and respect. Beginning from such assumptions, he sets out to produce a plausible blueprint for an affirmative answer to the question of political stability.

The central political principles of "A Theory of Justice" remain constant in "Political Liberalism," but the problem of stability gives them a new shape. Rawls now urges that we must not attempt to ground political principles in any doctrines, whether metaphysical or epistemological or religious, that are controversial among the religious and secular views of life that reasonable citizens hold. So, for example, we would be ill advised to base our political principles on the idea of the immortal soul, or the idea of "self-evident" truth, since many citizens do not accept such ideas. We can, however, Rawls thinks, argue for political principles in a thinner way, using ethical notions that are not inseparable from controversial religious doctrines.

Political principles, so understood, will not be separate from the rest of what religious and secular citizens believe. Instead, they will constitute a realm of overlap among all the "comprehensive doctrines" in the envisaged society  at least all those that are "reasonable," by which Rawls means willing to respect the equal dignity of all citizens. Each religious or secular doctrine will accept the political principles, and the independent moral arguments that ground them, as one part or "module" in their overall view of life, though most at this point will connect them to deeper metaphysical ideas and arguments. At the same time, citizens will also endorse the political conception as the basis for a mutually respectful and reciprocal life with one another. Thus the public realm is a realm in which we join hands and talk a common language. A key part of what we say in that language is that we agree to allow one another plenty of space to pursue the rest of what our sense of life's meaning requires of us.

I have described Rawls's ideas abstractly because they are, in fact, highly abstract. Rawls believed that philosophy's contribution to politics ought to be made in this way. Abstract models of an ideal can be extremely valuable as targets on which to fix our attention, as we try to make the world that way, rather than its current way or some worse way. Rawls knew that for this reason his book would strike many readers as "abstract and unworldly." But he then says: "I do not apologize for that."

He was right not to apologize, since he has produced a work that shows how the basic value of equal respect for persons can generate workable political principles that can help us live well together, without domination and with plenty of space for self-expression and personal commitment, in the heterogeneous and sometimes divided societies in which, today, all human beings live. More than any other modern work of political philosophy, "Political Liberalism" carries forward one's hope for humanity in an era of religious and ideological turmoil.

Ms. Nussbaum is the Ernst Freud Distinguished Service Professor of Law and Ethics at the University of Chicago, and the author, most recently, of "Liberty of Conscience: In Defense of America's Tradition of Religious Equality."