Urban legend has it that the man chosen by the BBC to deliver this year’s Reith Lectures was the real-world inspiration for a character in The Simpsons. Montgomery Burns, the desiccated and occasionally malevolent owner of the Springfield Nuclear Power Plant, is said to have been modelled, in his phy­sical characteristics if nothing else, on Michael Sandel, Bass Professor of Government at Harvard University. Many Simpsons writers have been Harvard alumni, and giving Mr Burns, one of Springfield’s least morally upstanding citizens, Sandel’s broad forehead and thin lips was a kind of Harvardian in-joke: for nearly 30 years now, Sandel has taught the university’s most popular undergraduate course – “Justice”.

Each year Justice, or Moral Reasoning 22, to give it its alternative title, a course in moral and political philosophy, draws more than 1,200 students, and the university has to requisition its largest lecture theatre to accommodate them. One of the most visited pages on the Harvard website carries a video in which Sandel addresses, without notes, a rapt audience on so-called “trolley problems”, imaginary dilemmas dreamt up by philosophers in order to get people to reflect on their intuitions about the relationship between action and intention. James Crabtree, now managing editor of Prospect magazine, was a teaching fellow (or graduate teaching assistant) on Justice between 2004 and 2006. He remembers the first time he saw Sandel lecture.

“It was pretty extraordinary. They were turning people away at the door. He’s a great lecturer – very engaging.”

A Harvard PhD candidate, Andrew Schroeder, also a former teaching fellow on Justice (the English-born political writer and blogger Andrew Sullivan is another), thinks the key to Sandel’s popularity with successive generations of undergraduates lies in his readiness to eschew the fantastic thought-experiments that are a staple of contemporary moral philosophy.

“His greatest strength, and one of the reasons Justice [is] so popular, is his ability to find real-world cases that show the depth and difficulty of issues in moral and political philosophy. Many discussions in moral philosophy are inspired by a single real-world example, and then move very quickly to focus on an abstract question. Sandel, though, can brilliantly marshal a huge range of examples to show how pervasive and difficult [an issue] is. That, I think, is what makes the class so popular and really inspires the students to work at the material,” Schroeder says. “I imagine Sandel reading the New York Times every day, cutting out articles that may have philosophical relevance and putting them in a file somewhere, to be summoned as necessary.”

Sandel doesn’t just read the New York Times, however; he is also a regular contributor to the paper’s op-ed pages, as well as to a number of other major American periodicals (including the Atlantic Monthly and the New Republic). Indeed, one assumes that it was his status as a “public” philosopher or intellectual – and not merely his reputation as a lecturer – that first caught the eye of the BBC.

For the past 15 years, Sandel has written as often for his fellow citizens as for his academic colleagues. His work, which has broached an impressively wide range of issues (from stem-cell research and affirmative action to the branding of sporting events and the use of commercial logos in schools), blurs the distinction between political commentary and political philosophy – and that is one of its strengths.

We met at Claridge’s, in central London, a couple of days after he had delivered “Markets and Morals”, the first of four Reith Lectures that will be broadcast on Radio 4 this month. He was dressed casually in a black polo neck, grey woollen slacks and expensive sneakers, like an East Coast academic in a Woody Allen movie.

Sandel sees his obligations as a philosopher as being continuous with his responsibilities as a citizen. For him, political philosophy is engaged or it is nothing. “The responsibility of political philosophy that tries to engage with practice is to be clear, or at least accessible – clear enough that its arguments and concerns can be accessible to a non-academic public. Otherwise, it’s not possible really for political philosophers to generate debate that could possibly challenge existing understandings.”

What is striking about this conception of the task of the public philosopher is just how ambitious it is. Where, for many of his contemporaries, the job of the philosopher is merely to tease out the abstract principles underlying public debate and deliberation, for Sandel it is to intervene in the debates themselves. “Public philosophy is set apart from academic political philosophy, in that it means not only to be about prevailing practices and assumptions, but also to address them,” he says. “To address fellow citizens about them and to try to provoke discussion and critical reflection among the public generally. So that political philosophy isn’t only about public things, but engages public things and, if it’s successful, reorients the way people relate to politics and the public realm.”

All of Sandel’s work, the academic treatises as much as the op-eds and magazine articles, circles obsessively around just this question: the nature and extent of the public realm. (The Reith Lectures are being delivered under the general heading “A New Citizenship” and culminate in a sketch of what Sandel calls “A New Politics of the Common Good”.) He traces these concerns back to a trip to southern Spain he took in the mid-1970s, at the end of his first term as a Rhodes scholar at Oxford, where he had gone to read for a DPhil after graduating Phi Beta Kappa from Brandeis University, in Massachusetts.“At the time, I thought I might pursue economics,” he tells me. “I was interested in welfare economics and the extent to which economic models could incorporate a concern for equality. I went to Spain with an economist, and we were going to try to work this out in a paper.”

One of his tutors at Oxford, Alan Montefiore, suggested to Sandel that he also take some books with him to Spain. He ended up taking four: John Rawls’s Theory of Justice, Kant’s Critique of Pure Reason, Robert Nozick’s Anarchy, State and Utopia and The Human Condition by Hannah Arendt. The idea was that Sandel would spend the days reading; he and his economist friend would work on their article in the evenings. Very soon, what he was reading by day caused the projected article on economics to unravel.

The “dismal science” of economics, he now thought, was excessively pessimistic about human beings, conceiving of them as little more than bundles of preferences and desires. This was a picture it inherited from utilitarianism, for which all moral and political principles are justified to the extent that they promote the greatest happiness of the greatest number. What Sandel took from reading Rawls and Kant was a “devastating and convincing critique of utilitarianism”.

In A Theory of Justice, which ignited a dramatic renewal of political philosophy in the US and Britain following its publication in 1971, Rawls had argued that by focusing exclusively on the promotion of the general welfare, utilitarians such as Jeremy Bentham and John Stuart Mill miss an equally significant dimension of moral and political life.

Political rights, for example, are important, Rawls said, not only because they tend to promote human happiness, but also because they protect individual human beings against being used as a means to some end or other, however desirable that end might be. Moreover, according to Rawls, basic rights and fundamental principles of justice could be derived in such a way that all reasonable people would endorse them, irrespective of their differing moral and religious beliefs – and that was crucial in modern, pluralistic societies such as the United States, which are characterised by profound ethical disagreements about the nature of the good life.

Though reading Rawls may have been what set Sandel on the path from economics to poli­tical philosophy, his academic reputation in the US was secured by his first book, Liberalism and the Limits of Justice (1982), based on his Oxford thesis – which was a full-frontal attack on the version of liberalism set out in A Theory of Justice. Under the influence of a notably heterodox group of philosophers at Oxford, which included Stuart Hampshire, Charles Taylor (with whom he recently co-taught a graduate class at Harvard) and Leszek Kolakowski, Sandel began to formulate deep misgivings about the Rawlsian model, which seemed to him to make the surrendering of the moral and religious convictions that people hold most dear a condition of access to the public sphere.

The effects of emptying public life of moral and religious discourse have been disastrous, Sandel tells me. “It’s contributed to a moral vacuum that has been filled by narrow, intolerant moralisms. It has allowed the Christian right to have more appeal than it might otherwise have had, precisely because the field was cleared.” Sandel’s argument is that political progressives, of whom he is one, should actively engage people’s deepest beliefs, rather than ignore them. “As recently as the 1950s and 1960s, in American politics it was the left, more than the right, which broached moral and religious themes. Think about Martin Luther King and his ‘Letter from Birmingham Jail’ – that drew unabashedly on Christian themes, as well as universal ones.”

Sandel thinks Barack Obama, for one, has recognised this. “He is trying to articulate a politics of the common good and, unlike a lot of politicians, particularly those to the left of centre, he does not shy away from engaging with moral and spiritual language. He has brought moral and religious sensibilities back into politics, against a background in which such themes have been monopolised by the Christian right. Progressives have reacted, not by engaging the Christian right, but by trying to keep morality and religion out of politics altogether.”

Sandel’s prescriptions for a “remoralisation” of the language of progressive politics appear to be striking a chord here, as well as in the United States. Leading politicians of all parties, including Ed Miliband, David Willetts and Dame Shirley Williams, were present to hear him deliver his first lecture, and the MP Jon Cruddas, one of the few philosophically curious occupants of the Labour back benches, thinks they are right to be listening.

“Sandel’s challenge is to the whole architecture of neoliberalism,” Cruddas tells me. “And not just to liberalism in an economic sense, but to liberalism more generally. What he is saying is that a particular conception of the individual is being challenged in the current economic crisis.”He is right: Sandel’s work is an uncomfortable reminder of what we lost when we threw in our lot with a vision of politics as little more than the pursuit of economic growth and the protection of individual choice.

The Reith Lectures by Michael Sandel begin on BBC Radio 4 on 9 June (9am)