OXFORD ― Can moral judgments be true or false? Or is ethics, at bottom, a purely subjective matter, for individuals to choose, or perhaps relative to the culture of the society in which one lives? We might have just found out the answer.

Among philosophers, the view that moral judgments state objective truths has been out of fashion since the 1930s, when logical positivists asserted that, because there seems to be no way of verifying the truth of moral judgments, they cannot be anything other than expressions of our feelings or attitudes. So, for example, when we say, “You ought not to hit that child,” all we are really doing is expressing our disapproval of your hitting the child, or encouraging you to stop hitting the child. There is no truth to the matter of whether or not it is wrong for you to hit the child.

Although this view of ethics has often been challenged, many of the objections have come from religious thinkers who appealed to God’s commands. Such arguments have limited appeal in the largely secular world of Western philosophy. Other defenses of objective truth in ethics made no appeal to religion, but could make little headway against the prevailing philosophical mood.

Last month, however, saw a major philosophical event: the publication of Derek Parfit’s long-awaited book “On What Matters.” Until now, Parfit, who is Emeritus Fellow of All Souls College, Oxford, had written only one book, “Reasons and Persons,” which appeared in 1984, to great acclaim. Parfit’s entirely secular arguments, and the comprehensive way in which he tackles alternative positions, have, for the first time in decades, put those who reject objectivism in ethics on the defensive.

“On What Matters” is a book of daunting length: two large volumes, totaling more than 1,400 pages, of densely argued text. But the core of the argument comes in the first 400 pages, which is not an insurmountable challenge for the intellectually curious ― particularly given that Parfit, in the best tradition of English-language philosophy, always strives for lucidity, never using obscure words where simple ones will do. Each sentence is straightforward, the argument is clear, and Parfit often uses vivid examples to make his points. Thus, the book is an intellectual treat for anyone who wants to understand not so much “what matters” as whether anything really can matter, in an objective sense.

Many people assume that rationality is always instrumental: reason can tell us only how to get what we want, but our basic wants and desires are beyond the scope of reasoning. Not so, Parfit argues. Just as we can grasp the truth that 1 + 1 = 2, so we can see that I have a reason to avoid suffering agony at some future time, regardless of whether I now care about, or have desires about, whether I will suffer agony at that time. We can also have reasons (though not always conclusive reasons) to prevent others from suffering agony. Such self-evident normative truths provide the basis for Parfit’s defense of objectivity in ethics.

One major argument against objectivism in ethics is that people disagree deeply about right and wrong, and this disagreement extends to philosophers who cannot be accused of being ignorant or confused. If great thinkers like Immanuel Kant and Jeremy Bentham disagree about what we ought to do, can there really be an objectively true answer to that question?

Parfit’s response to this line of argument leads him to make a claim that is perhaps even bolder than his defense of objectivism in ethics. He considers three leading theories about what we ought to do ― one deriving from Kant, one from the social-contract tradition of Hobbes, Locke, Rousseau, and the contemporary philosophers John Rawls and T.M. Scanlon, and one from Bentham’s utilitarianism ― and argues that the Kantian and social-contract theories must be revised in order to be defensible.

Then he argues that these revised theories coincide with a particular form of consequentialism, which is a theory in the same broad family as utilitarianism. If Parfit is right, there is much less disagreement between apparently conflicting moral theories than we all thought. The defenders of each of these theories are, in Parfit’s vivid phrase, “climbing the same mountain on different sides.”

Readers who go to “On What Matters” seeking an answer to the question posed by its title might be disappointed. Parfit’s real interest is in combating subjectivism and nihilism. Unless he can show that objectivism is true, he believes, nothing matters.

When Parfit does come to the question of “what matters,” his answer might seem surprisingly obvious. He tells us, for example, that what matters most now is 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.”

Many of us had already reached that conclusion. What we gain from Parfit’s work is the possibility of defending these and other moral claims as objective truths.

By Peter Singer

Peter Singer is professor of Bioethics at Princeton University and laureate professor at the University of Melbourne. Revised editions of his books “Practical Ethics and The Expanding Circle” have just been published. ― Ed.

(Project Syndicate)