I took part in a discussion at the Cambridge Science Festival last week on ‘The science of morality’. Others taking part were Ray Tallis, Pater Cave and Alasdair Coles. I have here mashed up my introductory and concluding comments.

Every year, I give a lecture to theology students on why I am an atheist. It is essentially a primer on why we don’t need God to explain the universe or to ground morality. And in talking of morality, I take up the argument, well-formulated by theologists like William Lane Craig, that ‘If God does not exist, objective moral values and duties do not exist’.

One of the points I make to my students is that even though they see the insistence on the need for ‘objective moral values and duties’ as an argument for the necessity for God, many of those most hostile to religion also insist on the need for ‘objective moral values and duties’. Except that they see science, rather than God, as the guarantor of objective values.So, the philosopher Patricia Churchland argues in her book Braintrust that it is a ‘false dilemma’ to claim that ‘either God secures the moral law or morality is an illusion’ because ‘Morality is grounded in our biology’. And Sam Harris, perhaps the most strident of contemporary critics of faith, attacks both religion and moral relativism, arguing that moral values are in reality moral facts and as facts they can be scientifically understood by studying brain and behaviour.

What I suggest to my theology students, and what I’m suggesting to you, is that it is the very search for ethical concrete is the problem. Not because I think that values are arbitrary or relative but because what makes values non-arbitrary is not that they are fixed in some transcendental sphere or that they are defined objectively by science but that they emerge through humanity’s collective judgment.

There is a wide spectrum of views about how science can illuminate our moral lives. At the ‘soft end’ of this spectrum is the idea that science can, at some level, illuminate our moral lives. Few would deny that. Unless we wish to imagine that values simply fall out of the sky, there must be some relationship between the kinds of creatures that we are, the kind of world in which we live, and the kinds of values under which we best flourish. And science helps us understand facts about the world and about ourselves.

That, however, is very different from the claims at the ‘hard end’ of spectrum; from claims that science can define morality, or that moral values are empirical facts explicable by science; that questions of morality devolve to questions of wellbeing, that wellbeing can be measured scientifically, primarily in utilitarian terms, and that there are in principle no moral questions for which science cannot provide an answer. That morality, in Sam Harris’s words, is ‘an undeveloped branch of science’.

It is not difficult, however, to imagine circumstances in which moral reasoning may require us to reject answers that scientific data or cost-benefit analyses suggest. Suppose that scientists were to discover that racial differences are a biological reality and that one race is cognitively inferior to another, and suppose that cost-benefit analysis showed indisputably that the best outcome for humanity was for that race to be enslaved by another. How should we morally respond? By insisting, I hope, that whatever facts science may discover about racial differences, and whatever may be the outcome of a cost-benefit analysis, there is a rational moral argument for treating all humans equally.

Why? Because all humans possess a certain integrity by virtue of being autonomous moral agents. The presumption of equal treatment derives from a profound insight about what it is to be human and no amount of empirical data about racial or other similar differences can alter that.

We can make similar arguments about torture, and a host of other issues. A moral answer may well be contrary to that suggested by scientific data, and there is nothing irrational about ignoring such data in making moral evaluations. It is simply that the logic of moral evaluations is different to that which undergirds the assessment of empirical data or utilitarian arithmetic. Or to put it another way, science cannot settle moral questions because we already have to possess certain normative assumptions (about what it is to be human, for instance) before we can interpret the empirical data; such assumptions certainly have to relate to facts (about the kind of creatures that we are, about the kind of world in which we live), but they cannot be reduced to facts.

Questions of morality do not have objective answers in the way that scientific questions do, but neither are they merely expressions of subjective desire or taste. To say that torture is wrong or that charity is good is qualitatively different to saying that light travels at 299,792,458 metres per second or that DNA is a double helix. It is also qualitatively different from making saying that ice cream is good or Justin Bieber awful. If everyone thinks that ice cream is bad or Justin Bieber good, I might privately despair. But if everyone were to believe that charity is bad and torture good, then there would be a tear in the fabric of society.

What makes values non-arbitrary, I suggested earlier, is that they emerge through humanity’s collective judgment. What do I mean by that? I mean that they are the products of the constant conversations we have with each other, within societies, across societies, metaphorically with the past and with the future. Moral questions may not have objective answers, whether revealed by God or by science, but they do have rational ones, answers rooted in a rationality that emerges out of social need. That rationality can only be discovered through exercising the human potential for rational dialogue, the potential for thinking about the world, and for discussing, debating and persuading others. Values can never be entirely wrenched apart from facts; but neither can they be collapsed into facts. It is the existence of human as moral agents that allows us to act as the bridge between facts and values.

This might seem a highly precarious foundation upon which to ground our moral world. But precariousness is the condition of being human. The desire to look to science to define moral values, just like the desire to look to God, emerges from a desperation to set values in what we may call ethical concrete. It is a yearning for moral certainty, a fear that without external authority, humans will fall into the morass of moral relativism. But just as we do not need the false certainty of a divinely-sanctified moral code, neither do we need the false certainty of a morality rooted in science.

Creating a distinction between facts and values is neither to denigrate science nor to downgrade the importance of empirical evidence. It is, rather, to take both science and evidence seriously. It is precisely out of the facts of the world, and those of human existence, that the distinction between is and ought arises, as does the necessity for humans to take responsibility for moral judgment.

Humans are moral beings living within a web of reciprocal rights and obligations created by our capacity for rational dialogue. We can distinguish between right and wrong, accept responsibility and apportion blame. The responsibility that falls upon humans is not simply for our individual acts. Humans are responsible, too, in the sense that it is up to us, and only up to us, to make moral judgments. We cannot alienate that responsibility to another being or another sphere or another process. Humans are, as Sartre put it, ‘condemned to be free’. To insist that science, or God, objectively defines moral values is to abandon our responsibility as human beings to make such judgments.

The human condition is that of possessing no moral safety net. No God, no scientific law, nor yet any amount of ethical concrete, can protect us from the dangers of falling off that moral tightrope that is to be human. That may seem a disconcerting prospect. Or it may seem an exhilarating one. In reality it is both. For that is what it is to be human.

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The images are Angela Palmer’s Self Portrait and Connections by AmandPreet Badhwar, one of the winners of the 2014 Brain Art competition.