I'm grateful to Sam Harris for the thoughtfulness and civility that he continues to show even when he responds to criticisms. I'm also conscious that there's much which he and I agree on.

For example, I am sceptical that science, by itself, can provide values or moral principles, but I don't see religion - any religion - as faring any better. I also agree with him in opposing popular, but crude, forms of moral relativism that counsel us to a sort of quietism about the practices of other cultures. I support Harris in speaking out against cruel practices wherever they take place.

Our disagreements are at a rather theoretical level, involving matters of meta-ethics and normative ethical theory. This can be dry stuff, and my gratitude to Harris extends to the way that he's made it interesting and opened up a popular debate.

There's a philosophical interest in sorting out these kinds of questions, but also a practical concern: the answers we give affect how we live our lives and judge our own life choices, what laws and political programs we decide to support, and how we teach the next generation of citizens. It's worthwhile trying to get these things right.

Harris claims that science can determine human values, but he goes about this in an odd way. Despite his protestations, he continues to rely on a fundamental value that is, by his own admission, not determined by science, the value of maximising general well-being - whatever "well-being" actually is.

In doing so, he deprecates the problem that we have no agreed conception of well-being, let alone a way of measuring it. Is it having various feelings such as pleasure or satisfaction, or having your preferences satisfied, or being able to do certain things if you wish, or leading a certain kind of life that is thought to be a flourishing one ... or what? After many hundreds of years, this is still debated.

Something similar applies to health, Harris says, so where's the problem? Surely, he argues, we can have a science of health. That may be true, though we can hardly aim to maximize health if the concept really is as fuzzy and contested as the concept of well-being.

As it happens, there's a vast literature in the fields of bioethics and health policy trying to nail down what health really is - inconclusively so far. Fortunately, medical practice can get by from day to day with more modest concepts than maximizing health, such as treating wounds and injuries, curing and preventing diseases, relieving pain, and so on.

I don't doubt that biomedical science finds out many truths about the functioning of the human body, and I don't doubt that the practice of medicine is a rational one, informed by, among other things, the findings of biomedical science.

Similarly, I don't doubt that we can engage in a rational practice when we criticize various laws, customs and social norms. Nor do I doubt that this can be informed by findings from various areas of science.

More generally, I believe that we can subject the whole phenomenon of morality to a process of rational study. What we can't do is try to maximize well-being if well-being can't be defined and measured.

Perhaps this problem can be overcome, but it's not clear that it's yet been overcome even with the concept of health, so that analogy is not reassuring.

Still, this is the least of the problems for Harris in The Moral Landscape . Deeper issues would arise even if we could agree on what well-being really is.

Even if we were, somehow or other, required to maximize well-being (however defined), Harris evidently agrees with me that this would not be an empirical finding. The requirement to maximize well-being is not something that is discovered by science or by any other area of empirical inquiry that the word "science" could be stretched to cover.

The best he can claim is that our laws, customs, policies, social norms, and so on could determined by the combination of this fundamental, presupposed value with information that we might obtain from science, and elsewhere.

That's not so implausible, but nor is it especially surprising. Harris seemingly wants to establish a stronger, more novel, claim, but I cannot see how he has done so.

In The Moral Landscape, and in responding to criticisms , he sometimes suggests that science depends on certain values, but that is very different from saying that it determines values. In any event, what does this amount to?

It's no doubt true that people are typically motivated to become scientists because they value truth, or at least discovering certain kinds of truths. Assuming, however, that science discovers truths about the universe, as opposed, say, to merely testing the "empirical adequacy" of various models, those truths are not established relative to whatever our values happen to be.

For example, a proposition such as "The Earth is between four and five billion years old" is simply true. It is not true relative to the values of geologists, whereas "The Earth is 6,000 to 10,000 years old" is true relative to the values of fundamentalist Christians.

If science discovers true propositions, as I accept it does, they are true, simpliciter, independently of whatever values scientists or others might happen to possess.

Contrast the situation of what it is rational for someone to do. Two well-informed people in very much the same situation may both reason perfectly well, yet one decides to act one way (perhaps making a home-cooked lunch) while the other decides to act the other way (going out to a restaurant).

They may act differently simply because they value different experiences, not because either is mistaken about any facts or because either has a defect in her reasoning process. Neither is "just wrong" to act in the way that she does, yet they act differently.

The decision of Alison to make a meal at home is not binding - on pain of being mistaken or irrational - upon Belinda. This situation is very different from that of the geologist and the fundamentalist Christian, where the latter really is mistaken about the age of the Earth.

When it comes to what it is rational for you to do, the answer is that it depends on what your values, goals, and so on actually are. As a first approximation, you can be said to act irrationally if you act in a way that defeats your goals, or stops you getting the outcome that you value.

Somebody who insisted that Alison and Belinda must act in the same way, or else one of them is "just wrong," would fail to understand what it is to act in accordance with ordinary practical rationality. If we are going to give Alison or Belinda advice on what it is rational for her to do, we will need to know what her values, goals, and so on, actually are.

Though there may be an "objective" answer as to what Alison should do, in the sense of an answer that really is correct for her, it will not be "objective" in the stronger sense of transcending her personal set of values, goals, and so on.

If someone who thinks there are objective answers in the first sense thereby concludes that there are objective answers in the second, she makes an elementary error.

I believe that Harris makes similar, if less obvious, errors. He observes the phenomenon of morality and concludes that it is possible to investigate the phenomenon scientifically. Very well. However far we want to stretch the idea of "science", I at least have no quarrel with the idea that we can make the phenomenon of morality a subject of rational investigation.

But this may lead to such findings, perhaps disconcerting ones, as that moral judgments by one person, such as Alison, are not rationally binding on others, such as Belinda, independently of Belinda's values, goals, and so on.

Indeed, nothing in any of the arguments put by Harris, either in his book or in responding to critics, goes anywhere near demonstrating that rational inquiry will find values that we are all bound to possess - on pain of being simply mistaken - and which are rationally bound to promote whether or not they are our values.

In short, rational investigation, or even science, may well discover many truths about the phenomenon of morality. But Harris is not entitled to assume that they will be moral realist truths, rather than moral anti-realist ones.

Nonetheless, given our actual values and goals, most of us can be given good reasons to act in such obviously "moral" ways as keeping promises, avoiding cruelty to others, settling disputes without violence, generally being helpful and cooperative, and paying for goods rather than stealing them.

Those reasons were discussed even in ancient times, and David Hume summarized a number of them in the eighteenth century. None of my arguments are intended to persuade you to abandon ordinary standards of kindness, honesty, cooperation, and the like.

If you have children, I hope that you'll teach such standards to them. People who do not internalise standards like that tend to be a danger to the rest of us, and we need to deter many of their actions in order to protect ourselves.

At the same time, it may be more difficult than it once was to offer good reasons to other people as to why they should abide by certain other traditional standards, such as those of piety and chastity. We really do need to look, on a case-by-case basis, at the various behavioural standards which we've inherited to see how far they continue to serve us well (given the underlying values and goals that we actually have).

Thus, I favour a practice of examining and criticizing moral claims. The practice can be a rational one, even "scientific" in a broad sense. It can extend to investigation of the observable phenomenon of morality, and to criticizing laws, customs and traditional standards of conduct.

All of this can be informed by empirical findings. But some of the truths it discovers may disconcert us. On the other hand, we have every prospect of showing why it is rational for almost all of us to act honestly, kindly, cooperatively, and so on, and to oppose such things as cruelty.

The moral scepticism that I favour undermines traditional moral systems that base morality in the commands of a god or in an idea of absolute moral good and evil, but it also undermines any idea that we are objectively required to do something as cosmic - and remote from our practical goals - as maximizing the well-being of all conscious creatures.

We can get by with more modest aims, such as each doing what we can, consistent with our other projects, to reduce the world's burden of suffering.

Harris and I agree that human beings do not require a belief in God, or the gods. Likewise, I suggest, we don't need to believe in anything so metaphysically outre as "objective values" or objectively binding moral judgments. We may, in fact, be better off - by ordinary, down-to-earth standards of "better" - without any of these illusions. But that's another story.

Russell Blackford is the editor-in-chief of the Journal of Evolution and Technology, and conjoint lecturer in the School of Humanities and Social Science at the University of Newcastle. He co-edited, with Udo Schuklenk, 50 Voices of Disbelief: Why We Are Atheists (Wiley-Blackwell, 2009).