For over two decades, the philosopher Michael Sandel has delivered a course of undergraduate lectures at Harvard with the unassuming title Moral Reasoning 22. It is now vastly popular, with annual attendance of more than a thousand. This book is an offspring of the course. Its origins are evident in its carefully crafted lucidity, its patient, teacherly tone and in its occasional professorial wisecrack. It is a reminder that in America, if no longer in Britain, educators do not have to play the fool in order to get a public hearing.

Justice is a pedagogical work in another, deeper sense. Sandel belongs to the tradition, dating back to ancient Greece, which sees moral philosophy as an outgrowth and refinement of civic debate. Like Aristotle, he seeks to systematise educated common sense, not to replace it with expert knowledge or abstract principles. This accounts for one of the most striking and attractive features of Justice - its use of examples drawn from real legal and political controversies. Should Florida shopkeepers have been permitted to take advantage of Hurricane Charley by hiking their prices? Is a wheelchair-bound girl eligible to be a cheerleader? Sandel explores many such cases in detail, as much for their intrinsic interest as for their ability to illuminate a point.

Even though the use of thought experiments is common in ethics, most philosophers prefer to simplify things by thinking up hypothetical, often bizarrely unreal scenarios. (In one classic example, the reader is invited to steer a runaway trolley car over this or that set of unfortunate bystanders.) Sandel indulges in a bit of "trolleyology" himself, but his general preference is for actual over hypothetical examples. This reflects his belief in the practical mission of philosophy. The point of ethical theory is to guide action in this world, not to elaborate principles for any possible world.

Sandel's ultimate target in Justice is the view, common to most modern liberals, that our basic framework of rights and duties should be neutral between competing visions of the good. This view has, he believes, desiccated political discourse in America, particularly on the left. Liberals take their stand on abstract principles of impartiality and choice, surrendering the more potent rhetoric of moral conviction to the fundamentalist right. Their language fails to connect with "the moral and spiritual yearning abroad in the land, or answer the aspiration for a public life of larger meaning". Sandel ends his book with a paean to Barack Obama, whom he sees as reviving the more idealistic progressivism of Robert Kennedy and Martin Luther King.

The modern liberal ideal takes two quite different forms, which Sandel tackles in turn. For libertarians, his first target, the state's remit is limited to securing individual rights. The moment it goes beyond this and tells individuals what to do with their possessions or bodies, it becomes paternalistic and coercive. Sandel invites all those tempted by this theory to think through its implications. Do we really wish to permit cannibalism (by consent, of course) or a free market in body parts?

Moreover, libertarianism reduces freedom to the ability to satisfy given wants, as opposed to the more exalted capacity to shape one's wants. This latter capacity - "autonomy", as Kant called it - requires for its realisation a basic minimum of wealth, education and civic engagement, none of which the libertarian can guarantee. Far from being neutral, the nightwatchman state perpetuates the rule of the strong over the weak.

The second strand of modern liberalism takes a more elevated view of the person and his needs. Its best-known advocate, John Rawls, argues that a just distribution of goods is one to which every member of a community would assent, were he ignorant of his place in it. Although more generous than anything envisaged by libertarianism, such a distribution does not violate the principle of neutrality, for it expresses the verdict of our true rational self. It is what we all would choose, could we be detached by a "veil of ignorance" from our actual interests and beliefs.

Thus, both strands of modern liberalism, for all their differences, have in common the principle that Rawls pithily summarised as "the priority of the right over the good". Both agree that rules of justice must derive solely from the formal properties of choice, independent of substantive moral ideals. What else could we hope for, living in societies irrevocably divided along religious and ideological lines?

Sandel rejects the priority of the right over the good. He reverts to the older idea, classically expressed by Aristotle, that to work out the just allocation of a good, we must first of all be clear about its nature. Flutes, for instance, were made to be played; therefore, the best flutes should go to those best able to play them, not to the wisest, the richest or the most beautiful. We cannot know how to allocate flutes until we know what flutes are for, what their telos is. And the same holds true of all social goods. We are thus drawn, ineluctably, into the disputed arena of values.

Sandel illustrates this thesis with the aid of many examples. Gay marriage, for instance, is often advocated by liberals (of both varieties) on grounds of moral neutrality. The state, they argue, has no business pronouncing on the desirability or otherwise of homosexual liaisons; its job is simply to underwrite individual choice, whatever that may be. But this line of argument, pressed to its logical conclusion, implies the licensing of polygamy, polyandry and every other variety of consensual cohabitation. In fact, it implies the complete privatisation of the marriage contract - a step favoured by some libertarians. If this is not what advocates of gay marriage want - and most of them do not - then it is clear that what really moves them is a belief in the equal dignity of homosexual relationships. In other words, their position is not neutral at all, but embodies a substantive moral claim.

Sandel's insistence on the inescapably ethical character of political debate is enormously refreshing - a riposte to the arid and evasive legalism of so much recent liberal thinking. Only one crucial problem remains. The reason modern liberals are so keen to put to one side questions concerning the good is that they think them unanswerable. They are, on the whole, moral sceptics - they hold that there is no such thing as moral truth, or at least none easily accessible to us. To tether questions of the right to questions about the good appears to them a recipe for civil war.

If Sandel's alternative is to convince us, he must show us that there is such a thing as moral truth, and that it is accessible to us. Failing this, his plea for a remoralisation of the public sphere looks like nothing more than nostalgia for a lost era of grand politics. And we have had enough of that in our time.

Edward Skidelsky is a lecturer in philosophy at Exeter University. His book, "Ernst Cassirer: the Last Philosopher of Culture", is published by Princeton University Press (£24.95)