WHEN he was a graduate student at Princeton in the early 1960s, Robert Nozick was known as the visiting professor's ordeal. However deeply the eminent guest had thought through the counter-arguments and rejoinders, young Mr Nozick could be relied on to spot a hole in the defences and work away at it until the structure of argument lay in ruins. This love of the chase stayed with him after he had become an academic celebrity in his own right. He relished pointing out flaws in his own arguments. In lectures, no question was too weird or off the point for him not to run with it to see where it led. Brilliance, restlessness and a distrust of authority marked all he touched in a distinguished professorial career, mainly at Harvard University, where he seldom taught the same course twice.

The book with which Mr Nozick burst on the public scene in 1974 was “Anarchy, State and Utopia”, a work of political thought that is acknowledged as a modern classic. It opens with the words, “Individuals have rights, and there are things no person or group may do to them (without violating their rights).” Given that, Mr Nozick immediately asked, what might states and their officials legitimately do? His answer was very little.

Against the pure anarchist, he allowed for a minimal state with a monopoly on legitimate punishment, provided it gave citizens adequate protection in return. But against anyone looking to the state to lessen material inequalities in the name of fairness he mounted a formidable challenge. No imposed pattern of property holding, he argued, is stable without continual interference in people's lives. The only just patterns of property distribution are those that derive by voluntary gift or trade from earlier just patterns. How can we be sure that the original pattern was just? Not because it was fair or unfair, but because it was justly acquired. In a neglected third part, Mr Nozick sketched a Utopian vision of like-minded groups freely forming communities in a minimal state that was “inspiring as well as right”.

No part of this impressive structure was (or is) without critics, particularly on the vexing issue of what makes original acquisitions just. Yet in the vigour of its arguments, the punch of its formulations (taxation is “on a par with forced labour”; “to each as they choose, from each as they are chosen”) and the breadth of its attack, the book had an impact far beyond the academic world. It is doubtful if many who bought it read it through. But its overall thrust—that a just distribution of property need not be fair—found welcoming ears.

It is hard to overestimate the part that “Anarchy, State and Utopia” played in restoring intellectual confidence among American conservatives and establishing a consensus in favour of free-market liberalism. Mr Nozick's book was the philosophical flank of a successful three-sided offensive against prevailing left-liberal ideas, the other two being Milton Friedman's economic work and the writings on politics and foreign affairs of the neo-conservative New York intellectuals grouped around the magazine Commentary.

In universities, teachers set Mr Nozick's book for undergraduates beside “A Theory of Justice” (1971) by John Rawls, a defence of principles underlying redistributive schemes that were the primary target of Mr Nozick's attack. Mr Rawls's classic account of justice as fairness had almost at a stroke rescued political thought from the doldrums, making it serious and interesting again. Mr Nozick made it lively: there were now two intellectually respectable versions of liberalism—left and right.

Mr Nozick, for all that, was not happy to be pigeon-holed. A socialist when growing up in Brooklyn, he decided the case against capitalism did not work. (Marxist exploitation, he later wrote, was mainly exploitation of people's ignorance of economics.) Though he chided fellow conservatives for not taking individual rights seriously enough, he refused to be called a libertarian. He taught political courses—on the Russian revolution, for example—but with colleagues he preferred talking philosophy. A rare public stand came in 1997 when he joined Mr Rawls and four fellow philosophers in a brief before the Supreme Court arguing that the law should let sane, terminally ill patients end their lives.

After his first book, he turned to pure philosophy, joking that he did not want to write “Anarchy, State and Utopia II”. In 1981 came “Philosophical Explanations”, which contains a famous chapter asking a seemingly bootless question, “Why is there something rather than nothing?”, as well as chapters on personal identity and on free will. It is best remembered for an ingenious argument against scepticism, and for a dispositional account of knowledge as true belief that would reliably stick with the truth (or self-correct) as relevant circumstances changed. In “The Nature of Rationality” (1993), he returned to Newcomb's paradox, an enduring puzzle about decision-making, and he suggested that rational-choice theory could be made less narrow and artificial if it included what he called symbolic utility: things matter to us, he recognised, for the values or emotional commitments they express, not simply for their practical or material usefulness.

Many such themes—and more—appear in “Invariances”, which Mr Nozick completed and published in October last year. Though written in his usual clear style, it is in many ways a daunting book. It is rich in detail and breathtaking in sweep. But it is not, as Mr Nozick himself warns us, systematic. Based on his John Locke lectures at Oxford University in 1997 and on other lectures elsewhere, its five parts explore relativism and truth, objectivity, necessity, consciousness and ethics.

No section is without close argument and fertile suggestion. A standard reply to relativism—the claim that truths depend on a point of view—is that this claim itself is either absolute (and so self-refuting) or relative (and therefore of no general import). This is too quick, Mr Nozick thinks, and he takes relativism more seriously: it does not undercut itself if we identify its domain of application clearly. Humans are enough alike for relativism about politics and society to be implausible. (In passing, he throws out the idea that social relativism gains favour among people who do not like the political truths that confront them—his example is resistance to accepting the success of capitalism.) In physics, by contrast, he enlists quantum theory to support the idea that truth is not (always) timeless. On a broader tack, he treats truth generally in a pragmatist spirit as whatever underlies the serviceability of our beliefs about the world—the X-factor in them that contributes to our attaining our goals.

This curt, dense summary of a single chapter should alert readers to what faces them. Mr Nozick goes on to unpack the idea of objectivity in terms of invariance under transformations of different kinds, to downgrade the notion of metaphysical necessity, and to suggest an evolutionary function for consciousness and for morals. The scientific, mathematical and philosophical learning needed to follow the many themes in full—the 100 pages of footnotes should perhaps be left for a second or third reading—is stupendous.

Philosophers will probably fasten on the hard arguments—on truth or necessity, say—and pass over the future-science guesswork about, for example, evolutionary cosmology. Non-philosophers (including many scientists) will, alas, be at sea. This reviewer, with “Invariances”, felt like a social chess player accompanying a grandmaster down the tables at a simultaneous display, struggling to follow each game while listening to him explain how chess would work in six dimensions. Mr Nozick was dying of cancer as “Invariances” went to press. Might his executors consider a new edition, edited and introduced by a sympathetic expositor?

That said, any reader of “Invariances” can gain from it a feel for the scope and tenor of Mr Nozick's thought. He writes in a recognisably American tradition of pragmatism and scientific naturalism. One of philosophy's main jobs, he says at the start, is to prepare topics for empirical science to go to work on. Yet the science he is drawn to is the speculative, intuition-defeating sort. He loved puzzles and free-ranging argument too much, you feel, ever to surrender entirely to the constraining authority of slowly accumulated fact.

Philosophy begins in wonder, he writes at the end, with a silent nod to A.N. Whitehead. Indeed, Mr Nozick had a Romantic streak, both in his Utopian vision of society and in his conduct of philosophy. But this Byronic restlessness was the fault of his virtues: rare fluency and audacity—a fearless readiness to follow an idea where it led. Like any endeavour, philosophy needs explorers as well as mapmakers. As Mr Nozick liked to say, there is room for words that are not last words.

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