Derek Parfit's On What Matters has been the most eagerly awaited work in philosophy since Ludwig Wittgenstein's Philosophical Investigations. Drafts of its chapters - and those of its prototype, Climbing the Mountain - have been in circulation for more than a decade. Indeed, the book - if that is the correct term for two large volumes containing three distinct treatises and much additional matter - has already featured as the focus of numerous academic articles, conferences, blogs, an edited volume and a Facebook reading group. Scholars including Brad Hooker and Peter Singer have hailed it as the most important publication in moral philosophy since Henry Sidgwick's 1874 work The Methods of Ethics, which is no small praise given that the competition includes key works by figures as diverse as Friedrich Nietzsche, Oscar Wilde, G.E. Moore, W.D. Ross, Iris Murdoch, John Rawls, G.H. von Wright, Alasdair MacIntyre, Bernard Williams, Philippa Foot and T.M. Scanlon.

Parfit has not published much since his groundbreaking 1984 book Reasons and Persons, which The Sunday Times described as "something close to a work of genius". That work almost singlehandedly revived a relatively dormant area of philosophical investigation (the theory of normative reasons) while largely destroying the hopes of another (theories of personal identity, with Parfit arguing that what mattered was not identity but survival).

On What Matters returns to the first of these two themes, opening with a rigorous defence of the view that there exist objective facts that count in favour of certain courses of action regardless of our desires, and closing with an all-out attack on various sceptical moralities that have plagued philosophy ever since Thrasymachus told Socrates that justice is might. However, it is the material that lies between these two undertakings that has caused the most excitement.

In this largest part of the book, Parfit attempts to demonstrate that the best versions of three prominent ethical theories traditionally viewed as being opposed to each other actually converge. The theories in question are consequentialism, Kantian deontology and contractualism. It is widely held, for example, that Immanuel Kant's view rules out certain kinds of action no matter what their consequences. But if Parfit's thesis is right, then a significant part of the history of Western moral philosophy has rested on a mistake.

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Should the book become as influential as the stars guiding its arrival suggest, it could seriously alter the way that ethics is thought about and taught, not only in university philosophy departments but also in military academies, business schools, environmental programmes, political theory, religious studies and even at GCSE level.

Parfit painstakingly works his way through the most popular formulations of each view, revising them against counter-examples until they are each as tight as possible. The resulting theories are: a version of rule consequentialism according to which "everyone ought to follow the principles whose universal acceptance would make things go best"; a contractualist formulation of Kant's categorical imperative, according to which "everyone ought to follow the principles whose universal acceptance everyone could rationally will"; and a revised account of Scanlon's social contract theory, according to which "everyone ought to follow the principles that no one could reasonably reject".

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Parfit argues that all three converge to prescribe the same set of actions, despite their emphases on features of morality that are in prima facie tension. He takes these actions to be recommended by a "triple theory" that combines three properties shared by all three of the aforementioned principles: "An act is wrong just when such acts are disallowed by the principles that are optimific, uniquely universally willable, and not reasonably rejectable."

Accordingly, he argues, rival theorists have been "climbing the same mountain on different sides" to reach the same view of what matters, namely "that we rich people give up some of our luxuries, ceasing to overheat the earth's atmosphere, and taking care of this planet in other ways, so that it continues to support intelligent life". The triple theory, Parfit maintains, gives us overwhelming reasons to believe that this is the truth about what matters and then to act accordingly (with Parfit defending his claims about normative reasons and truths in the first and last parts of the book).

The theories may well converge on their recommendations, but to think that the actions that follow from them are all that matters is to already presuppose the truth of consequentialism. Suppose we all agreed and acted upon these recommendations. Would it not matter if some of us do so because we think it would be callous not to, while others do it because they think it would be self-defeating not to, because God commanded it, or because it would minimise overall suffering? Parfit's use of the phrase "things would go best" masks all sorts of important disputes about value.

A different worry is that of whether the three principles defended by Parfit really do recommend the exact same set of actions. In the final page of his responses to the commentaries by Scanlon, Susan Wolf, Barbara Herman and Allen Wood that are included in this two-volume work, Parfit allows that there may be various ways in which "the three parts of the Triple Theory sometimes conflict". However, he does not see this possibility as problematic, taking it to show only that we need to keep revising the theories until they become even more plausible. "If what seem the most plausible theories have very similar implications, this fact gives us reasons to believe that we are making progress, and that these are the theories that we should try to develop further, and revise," he writes.

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But this leads to the opposite worry, that the further removed these revised versions become from their traditional formulations, the less exciting their convergence will be. Indeed, given that Parfit takes convergence with the most plausible formulations of principles of other theories to be a criterion of the very plausibility of any given principle, the surprising result would be the one in which the most plausible versions of each theory did not converge. It would seem there's no success like failure, and that failure's no success at all.

Why should we take similar implications as a sign of moral progress? In his private correspondence, Sidgwick expressed the fear that if the Universe and the individuals it contains are impermanent it would be absurd to talk of absolute good and evil, as he did. Parfit, both his 1984 book and the present one, shows this fear to be misplaced in different ways. He is, however, plagued by a parallel worry, namely that deep moral disagreement among mortals would be evidence for the view that morality is an illusion.

But it is not clear to me why we should think this, given the range of plausible explanations for the historical development of mutually exclusive theories. Parfit does not explore alternative non-sceptical explanations, such as that made famous by Alasdair MacIntyre in After Virtue: A Study in Moral Theory. Incidentally, Parfit does not concern himself with virtue ethics at all, presumably because he thinks that its only action-guarding version collapses into a form of motive consequentialism that advises us to act upon whatever motives have the best consequences. It would have been nice to have seen the argument, though.

Whatever one makes of its conclusions, On What Matters is an achievement that should make us rethink the academic values of the research excellence framework-driven "publish or perish" age we live in. But whether it will change the way we think about morality remains to be seen.

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The Author

Derek Parfit says he is unusual in the amount of time and energy he has devoted to just two things: philosophy and photography. Yet at school, he specialised in history, although largely to avoid Latin and Greek.

While completing his bachelor's degree in modern history at the University of Oxford, Parfit successfully applied for a Harkness Fellowship to spend two years in the US at Harvard and Columbia universities. Without that Fellowship, he believes, his move from history to philosophy might not have been possible. On his return to the UK in 1967, Parfit won a Prize Fellowship at Oxford, and ended up in the uncommon position of being an academic philosopher without a degree in the subject, and without having had to study formal logic, which he says he would have hated.

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Parfit's main hobby is architectural photography, and he says he is still puzzled by the intensity of feeling he has for architecture. In his view, Venice and St Petersburg have the most beautiful buildings, and he has visited both many times. "I also love the avenues in the French countryside," he says, "perhaps because the trees are like rows of pillars."