Many years ago, the philosopher Thomas Nagel wrote a famous paper called "What Is It Like to Be a Bat?". Amartya Sen has written a book that has something to do with a rather similar question: what is it like to be a human being? Nagel's paper was not much concerned with bats. Sen's book is profoundly concerned with human beings. Its aim, he says, is to clarify how we can proceed to address questions of enhancing justice and removing injustice.

It is a book dedicated to the discipline of practical reason. Reasoning for Sen - public reasoning - is a moral and political imperative. "The avoidance of reasoned justification often comes not from indignant protestors but from placid guardians of order and justice. Reticence has appealed throughout history to those with a governing role, endowed with public authority, who are unsure of the grounds for action, or unwilling to scrutinize the basis of their policies." Reason, he avers, is central to an understanding of justice, even in a world that contains much unreason; "indeed, it may be particularly important in such a world".

It is a book of reason, and also a book of ideas - it is thick with ideas - but it cleaves throughout to the real world. There is a sense of powerful feelings held in check, the pressure eased every so often with a choice epithet like "placid". Words go off like depth charges in these pages. Sen may be a paragon of patient exposition, but he is not placid. He is scrupulously careful never to raise his authorial voice; he is philosophically generous to friend and foe alike. Yet there is no mistaking his commitment. There is no irreducible conflict between reason and emotion, as he argues himself, "and there are very good reasons for making room for the relevance of emotions" - a characteristic formulation.

The Idea of Justice is the very opposite of an emotional tirade, but it is in the best sense an argumentative work, and a deeply engaged one. Amartya Sen's passionate intellectual project is to identify remediable injustice, in order to remedy it, without further ado. He does not shrink from specifying the injustices he has in mind, even where they are close to home. In fact, he is militant against them.

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The demands of justice summon action, he insists, "whether we fight oppression (like slavery, or the subjugation of women), or protest against systematic medical neglect (through the absence of medical facilities in parts of Africa or Asia, or a lack of universal health coverage in most countries in the world, including the United States), or repudiate the permissibility of torture (which continues to be used with remarkable frequency in the contemporary world - sometimes by pillars of the global establishment), or reject the quiet tolerance of chronic hunger (for example in India, despite the successful abolition of famines)".

Right from the beginning, The Idea of Justice has the stamp of a classic, at once timely and timeless, responsive and inclusive, moderate and pellucid: "'In the little world in which children have their existence,' says Pip in Charles Dickens' Great Expectations, 'there is nothing so finely perceived and finely felt, as injustice.' I expect Pip is right: he vividly recollects after his humiliating encounter with Estella the 'capricious and violent coercion' he suffered as a child at the hands of his own sister." But the strong perception of manifest injustice applies to adult human beings as well.

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"What moves us, reasonably enough," says Sen, "is not the realisation that the world falls short of being completely just - which few of us expect - but that there are clearly remediable injustices around us which we want to eliminate." And so we are launched. Strong as they may be, such perceptions are often incoherent, disorderly, or down-right confused; they lack discipline or direction; they fizz and fizzle, without effect. What they need is an equally strong dose of practical reason. Who better to administer it? Sen seeks to harness what moves us; to provide an intellectual underpinning; to explain ourselves to ourselves.

That makes for a work of rare distinction, a lesson in acuity, humanity and sagacity. It is always clear, although not always easy - often intricate, according to one commentator, but never worthy. Neither is it dull. On the contrary, one could say with Sen that it is reasonably exciting. Its international address is very striking. "Parochialism" is plainly one of his pet hates, and another of those little depth charges. One of his earlier books is called The Argumentative Indian (2005). Sen himself was born in India in 1933; he grew up there. While he has become an eminent cosmopolitan, he remains an Indian citizen. In the light of his subsequent preoccupations, not to speak of the searing memory of childhood he appropriates so convincingly from Dickens, it is tempting to speculate on the significance of his own boyhood experiences: he witnessed the 1943 Bengal famine and he has written elsewhere of the devastating effect of seeing an itinerant Muslim die after being attacked by a Hindu mob. The argumentative Indian is the bane of the parochial.

Perhaps the most impressive thing about the book is its intellectual reach. It could be prescribed as a set text across the entire curriculum of the university, regardless of disciplinary boundaries. If that is too tall an order in the age of the reading pack and Wikipedia, then the introduction and the conclusion may serve. Like many a great book, it is surely destined to be cannibalised, summarised, excerpted, digested, filleted and force-fed as gobbets for years to come. A seasoning of reasoning may be all to the good. Nigella bites. Amartya nibbles.

The Idea of Justice is in every sense an exemplary work. In its very nature it stands interestingly athwart certain contemporary trends. First of all, running counter to impoverished notions of efficiency of resource allocation, it is a hymn to co-teaching. Amartya Sen is one of the great co-teachers of our time. His experience at the universities of Harvard, Oxford and Cambridge is now unrepeatable: he mentions teaching jointly with John Rawls and Kenneth Arrow, with Robert Nozick, with G.A. Cohen, Ronald Dworkin and Derek Parfit, with Michael Sandel, with Thomas Scanlon, and many others. His testimony to the benefits of this experience for his own intellectual and professional development is decisive.

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Second, and closely linked, it is a paean to conversation, or as Sen might say, chatter. Citing Piero Sraffa and Ludwig Wittgenstein, and more modestly his own "chatty friendship" with Bernard Williams, he believes in the sovereign importance of free-range conversation among cosmopolitan intellectuals of different bents and specialisms. Finally, following Sen himself, one cannot but remark that this magnificent book is a distillation of a long period of teaching, and chatting, and writing, and thinking. According to its author, it could not have been written sooner; it has matured over some 25 years, which is to say three to five research assessment exercise cycles. Now we have it. If democracy is government by discussion, as Walter Bagehot said, humanity is reason by argumentation, as practised by Amartya Sen.

THE AUTHOR

Amartya Sen is Lamont university professor and professor of economics and philosophy at Harvard University. It was perhaps expected for Sen to become an academic, he says in his Nobel prize autobiography: "I was born in a university campus and seem to have lived all my life in one campus or another."

From 1998 to 2004, Sen was the master of Trinity College, Cambridge, and was the first Indian academic to head an Oxbridge college. He has taught at the universities of Oxford and Delhi and also at the London School of Economics. Using his expertise on welfare economics, he was formerly honorary president of Oxfam and is now its honorary adviser. Sen has been awarded many prizes for his work on welfare economics, but is perhaps best known for his Nobel prize, which he was awarded in 1998.