It is now generally accepted that the roots of our ethics lie in patterns of behaviour that evolved among our pre-human ancestors, the social mammals, and that we retain within our biological nature elements of these evolved responses.

We have learned considerably more about the nature of these responses, and we are beginning to understand how they interact with our capacity to reason. Many philosophers now recognize the relevance of this work to our understanding of ethics.

The term "sociobiology" was coined by E.O. Wilson in his 1975 book Sociobiology: The New Synthesis, a pioneering multi-disciplinary study that aroused a storm of controversy because Wilson applied theories about the evolution of social behaviour - in organisms as different as bees and chimpanzees - to humans.

Wilson made a significant contribution to our understanding of human nature, but in writing about ethics, he committed a fallacy common among scientists who turn their attention to that field.

Wilson's misunderstanding of the import of his own work for ethics provided the impetus for me to write The Expanding Circle some thirty years ago, both to explain the fallacy he was making, and to demonstrate that despite it, Wilson's approach does help us to understand the origins of ethics. Hence that book followed Wilson's writings receive closer scrutiny than the work of any other scientist.

Those parts of sociobiology that relate to human beings are now referred to as "evolutionary psychology". Although the application of sociobiology to human beings was fiercely opposed by some researchers, the development of evolutionary psychology has had a calmer reception.

To that extent, the rebranding has been a resounding success, although one could also argue, less cynically, that the growing acceptance of evolutionary psychology is due to the merits of the studies it has produced, rather than the change of name.

If thirty years ago most philosophers were disdainful of what scientists wrote about ethics, that may have been because some scientists suggested that the scientific breakthroughs they were making could be, not merely relevant to, but a substitute, for the thinking that philosophers do about ethics - what I refer to in chapter 3 of The Expanding Circle as "The Takeover Bid."

It is a mistake to believe that scientific findings could be a substitute for the kinds of thinking that philosophers do about, and in, ethics. I hope that the new edition of The Expanding Circle will help to make it clear (again!) why such attempts are bound to fail, and philosophers are right to continue to reject such attempts to annex ethics or moral philosophy; while they should welcome scientific help in understanding the origins and nature of ethics as a human phenomenon.

But for now let me put aside the question of whether we can separate those moral judgments that we owe to our evolutionary and cultural history, from those that have a rational basis. Instead, I want to inquire further into the extent to which a moral judgment can have a rational basis.

In re-reading The Expanding Circle , I can see how ambivalent I was about the idea of ethics being objectively true and rationally based. I wrote that reason leads to progress in morality and I insisted that reason is not limited to the negative task of rejecting custom as a source of authority.

On the contrary, I argued that reason leads to the principle that "one's own interests are one among many sets of interests, no more important than the similar interests of others." Moreover I said that this truth is "eternal and universal, not dependent on the existence of human beings or other creatures with preferences," although without such beings it would have no application.

Yet I went on to say that the idea of "objective values" or an "objective moral reality" is too "queer" and too full of problems to be used to support alternatives to the view that, given that one's own interests are no more important than the interests of others, the right thing to do is maximally to satisfy the preferences of all those affected by our actions.

I therefore claimed that these alternatives - for example, the view that it is always wrong to kill an innocent person, no matter how many other innocent people may die if we refrain from killing one - should be regarded as the subjective preferences of the person who holds them.

If we do that, of course, they can be taken into account when we decide what will maximally satisfy the preferences of all those affected, but they are taken into account on terms set by the one who seeks to maximize the satisfaction of preferences - that is, by the preference utilitarian.

I no longer believe that this argument succeeds. The judgment that "one's own interests are one among many sets of interests" can be accepted as a descriptive claim about our situation in the world, but to add that one's own interests are "no more important than the similar interests of others" is to make a normative claim.

If I deny that normative claims can be true or false, then I cannot assert that this claim is true. It too could be treated as just one preference among others - except that now there is no basis for saying that we ought to maximize the satisfaction of preferences.

Moreover, even if others accept that their interests are no more important than the interests of others, this is not enough to justify the conclusion that we ought to satisfy everyone's preferences to the greatest possible extent.

In saying this, I do not hold that my own interests are of greater importance than the interests of others, and I do not violate the widely-accepted requirement that moral judgments must be universalisable.

The denial of objective truth in ethics thus leads not, as I had tried to argue, to preference utilitarianism as a kind of metaphysically unproblematic default position, but to scepticism about the possibility of reaching any meaningful conclusions at all about what we ought to do.

The only conclusions we could reach would be subjective ones, based on our own desires or preferences, and therefore not ones that others with different desires or preferences would have any reason to accept.

I was reluctant to embrace such sceptical or subjectivist views in 1981, and that reluctance has not abated over the intervening years.

What then is the alternative? In The Methods of Ethics, Sidgwick investigated a range of ethical intuitions and principles, and winnowed them down to three "intuitive propositions of real clearness and certainty." It may be helpful to state them briefly, so that they can serve as examples of what moral truths might look like:

(1) The axiom of fairness or equity: "if a kind of conduct that is right (or wrong) for me is not right (or wrong) for someone else, it must be on the ground of some difference between the two cases, other than the fact that I and he are different persons."

(2) The axiom of prudence: We should have "impartial concern for all parts of our conscious life ... Hereafter as such is to be regarded neither less nor more than Now."

(3) The axiom of universal good: "each one is morally bound to regard the good of any other individual as much as his own, except in so far as he judges it to be less, when impartially viewed, or less certainly knowable or attainable by him."

Sidgwick argued that these axioms or "rational intuitions" are truths in much the same way as the axioms of mathematics are truths. The view that there could be truths of this kind in ethics was widely held at the time, and continued to be accepted by philosophers who came after Sidgwick, such as G.E. Moore and W.D. Ross.

In the 1930s, however, logical positivism became dominant in English-language philosophy, and for logical positivists, truths must either be tautologies, that is, true in virtue of the meanings of the terms used, or they must be empirical.

Mathematical truths, on the positivist view, are tautologies. They unpack the meanings of the terms used, or perhaps of some axioms that are not in themselves either true or false.

But ethical axioms that offer genuine guidance cannot be tautologies. Nor, however, can they be empirical truths (for reasons given in chapter 3 of The Expanding Circle ), and in any case, for the logical positivists, if truths are empirical, there must be a way of verifying them.

If a proposition is not a tautology and there is not, even in principle, a method of verifying it, then logical positivism held it to be meaningless. Sidgwick's axioms fell into that category.

Although the era of logical positivism has passed, the idea of truths that are neither tautologies nor empirical can still seem puzzling. Recently, however, Derek Parfit has written a remarkable defence of normative truth.

In On What Matters , Parfit argues that unless we are to fall into scepticism about knowledge as well as scepticism about ethics, we must accept that there are normative truths about what we have reason to believe, as well as about what we have reason to want, and reason to do.

Consider, for example, the statement: "When we know that some argument is valid, and has true premises, we have decisive reasons to accept this argument's conclusion." That statement, Parfit argues, is neither a tautology, nor an empirical truth. It is a true normative statement about what we have reason to believe.

In chapter 4 of The Expanding Circle , I echo Mackie's doubts not about the possibility that "to-be-pursuedness" or "not-to-be-doneness" could be built into the very nature of things. Parfit points out that Mackie's difficulty here was in understanding how any beliefs about the world could necessarily be motivating for anyone who believes them, no matter what wants or desires a person may have.

This was my problem too. I may believe that by donating to Oxfam a sum that will not significantly affect my life for the worse, I can save the lives of ten children and greatly reduce their suffering and that of their families. But this belief may not motivate me to make the donation, because I might not care about the children of strangers.

But, Parfit says, whether a belief gives us reasons to act in a certain way is a normative question, and whether it motivates us to act in that way is a psychological question.

Because many people may respond to this example by saying that if I don't care about the people Oxfam is helping, I have no reason to donate to Oxfam, here is another example in which it is harder to deny that I have reason to do what I do not desire to do.

I am about to spend a month on a remote island where there are no dentists when I detect the early signs of a toothache coming on. On the basis of past experience, I believe that if I don't go to the dentist today I am very likely to suffer an agonizing toothache all next month, which will prevent me enjoying what will otherwise be a rare opportunity for me to relax and enjoy the natural beauty of the island. If I do go to the dentist today, I will suffer mild discomfort for less than hour. My knowledge that I will suffer an agonizing toothache all next month if I do not go to the dentist gives me a reason to go to the dentist today. It would be irrational of me to ignore the pain I will suffer if I do not go.

This is consistent with Sidgwick's axiom of prudence, which says it is irrational not to have impartial concern for all parts of one's conscious life, but even a weaker form of this axiom, allowing one to have somewhat less regard for the more distant future, would be a sufficient basis for adjudging me irrational if I do not go to the dentist today.

But note that nothing has been said about my present desires. Perhaps I am the kind of person who is more influenced by what will happen to me now, or in the next few hours, than what will happen to me tomorrow or next week.

Hence right now, when I am standing in front of my dentist's office, what I most want is to avoid anything even slightly unpleasant today. Intellectually, I know that next week, when I am in agony and my island sojourn is being ruined, I will regret this decision, but at the moment that fact has no impact on my desires. The fact that next week's agony does not motivate me to take steps to prevent it, however, does not vitiate the claim that I have a reason to take such steps.

If we gain acceptance of the claim that there are objective reasons for action only by granting that even those who fully acknowledge the existence of a reason for doing something will not necessarily be motivated by it, have we won only a pyrrhic victory?

We may be able to say that there are objective reasons for you to give to Oxfam, but if we cannot motivate you to give, the poor will be no better off. Nevertheless, if we can accept the idea of objective normative truths, we do have an alternative to reliance on everyday moral intuitions that, according to the best current scientific understanding, are emotionally based responses that proved adaptive at some time in our evolutionary history.

The existence of objective moral truths allows us to hope that we may be able to distinguish these intuitive responses from the objective moral truths - that is, from the reasons for action that all rational sentient beings would have, even rational sentient beings who had evolved in circumstances very different from our own.

Peter Singer is Professor of Bioethics at Princeton University and Laureate Professor at the University of Melbourne. This article is an edited extract from the new edition of The Expanding Circle: Ethics, Evolution, and Moral Progress, which has just been published by Princeton University Press.