Lewis Wolpert, a man of science, and Richard Harries, a man of religion, face off in a rehearsal of their positions for tonight's inaugural 'Controversial Thesis' debate at the National Portrait Gallery. The next event in this series sponsored by The Times Higher will be on April 22, when Colin Blakemore and Gill Langley will consider 'two mice and half a rat per person is a fair price to pay for academic progress'

Proposed by Lewis Wolpert
It seems most unwise to choose our ethics on the basis of religious beliefs. Not that ethical views based on religion are always unsatisfactory, but religion is based on faith and mysticism, and so is an unreliable basis for ethical decisions, particularly when it comes to the applications of science. It is also based on absolute authority and so is in clear opposition to democratic principles. Just recall the case of Abraham, when God insisted that he sacrifice his son without giving any reason whatsoever. No wonder religious beliefs have been the cause of bigotry, hatred, deaths, including suicide, and wars.

While some of the ethical ideas deriving from religion are perfectly acceptable, particularly those dealing with our behaviour towards others in a religious community, on some issues there are serious omissions. In the Ten Commandments there is an emphasis on children honouring their parents. This minimises the role parents have in looking after their children.

Religion also says nothing about the care of the mentally ill and how we should try to deal with the associated stigma. In relation to politics, what has religion to offer with respect to how we treat women or exploit workers?

A rather different set of ethical beliefs relates to the very nature of life, the issue of when life begins and where it comes from. For the ancient Egyptians, it was the sun god who was the creator of the germ in women and the seed in man. Plato thought the foetus a living creature as soon as it moved. Aristotle was sure it had a soul, although it entered the female foetus later than the male. St Augustine put entry of the soul at 40 days for males and 80 for females, but then later set 40 days for both sexes. In 1745, Canigiamilla advocated caesarean section to ensure baptism.

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Doctors at the Sorbonne in Paris even recommended intrauterine baptism by means of a syringe. This is the origin of the religious idea that the embryo is human from the moment of conception.

So, from religion comes the idea that life is associated with souls, the belief that there is a mystical life property that cannot be accounted for by physics and chemistry or by any other science. The status of the early embryo from the moment that the egg has been fertilised by the sperm is that of a human being according to the Christian religion, most particularly the Roman Catholic Church, which sees all human life as being created by God in His own image. The basis for these religious convictions is far from clear, and is of relatively recent origin, but it is of great importance because it could prevent in vitro fertilisation, prenatal diagnosis for genetic illnesses and the use of embryonic stem cells, which could help with many illnesses.

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Religious belief can also prohibit termination of a pregnancy, even if the child will be severely handicapped. But it is the wellbeing of the child that should determine the related ethics. There is, in fact, no biological basis for believing the early embryo is a person if only because the fertilised egg can develop into more than one person.

I would argue that the embryo is only a person when the baby can survive outside the womb with minimal technical support. Even reproductive cloning raises no new ethical issues, although it should be banned because the risk to the child is severe.

Moreover, the strong stand taken against abortion by some elements of the Christian church can have serious effects. Homicide by young men in the US peaked in 1993 and has steadily decreased since. Careful analysis has shown that it is because of the legalisation of abortion: unwanted children are more likely to commit crimes, and women who have abortions are already in deprived circumstances. Without the key legal decision in the US in 1973 that permitted abortion, more violent children would have reached their peak crime years in about 1991. It is estimated that as much as 50 per cent of the recent drop in crime in the US is due to legalised abortion.

Ethical decisions must be based on humanism and democratic principles rather than religious belief. Humanists value personal freedom and choice as long as they do not interfere with anyone else's freedom, happiness or security.

Lewis Wolpert is professor of anatomy and developmental biology at University College London.

A few tickets are still available for tonight's (Thursday's) debate. Call 0207 306 0055 ext 216

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Opposed by Richard Harries
I do not believe that religion provides the only basis for ethics. Furthermore, the Christian natural law tradition, with which I have much sympathy, holds that all people, whether they have a religious view or not, are capable of making rational moral judgements. If we take the question of research on embryos, for example, there will be much in common in the process of reflection between those who argue from a religious perspective and those who don't. The concept of "respect" for the early embryo, put forward by the Warnock committee and, by implication, enshrined in the 1990 Human Fertilisation and Embryology Act, is one that people of all faiths and of none are likely to share. In short, the early embryo isn't to be treated simply as a piece of valueless jelly. It is a potential person.

An act of judgement, weighing up two immeasurables - respect for the early embryo against the great benefits that are likely to come from research on it for a whole range of people suffering from serious debilitating diseases - has to be made.

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Similarly, for human reproductive cloning, much of the ethical reasoning on issues such as the identity and self-worth of any cloned child will be similar for those who have a religious faith and those who don't. Both religious and non-religious will prioritise the welfare of any potential child and will evaluate the issues in this light.

Where a great divide does come in is between those who believe that the early embryo is to be accorded the full rights of a human being. This is the Roman Catholic position and one that is shared by a number of other Christians. This view could be shared by an atheist or agnostic, although this is unlikely, or by someone who believed in reincarnation.

Religious believers of many kinds and those who have no religious belief will agree that a human being should never be treated merely as a means to an end. However, there is a big difference in what are regarded as the facts of the case. Those who take one of the range of positions mentioned above believe that the early embryo is either a person or, in the more nuanced view of many Roman Catholic theologians, might be a person. There is a difference not so much in ethical judgement but about what is believed to be the case.

This is not a view I share. I cannot regard every abortion as morally wrong. I believe that research on early embryos, up to 14 days, under controlled conditions, if there is no other way of obtaining the necessary research results, is justified in the light of the great potential benefits that are likely to be achieved. Nevertheless, I do not see how we can rule out the Roman Catholic view from having a proper place in any ethical discussion or treat it as superior or inferior to a secular-based ethic.

In a democratic society, there will usually be much overlap in the ethical reasoning between a secular and a religious ethical position. Sometimes there will be differences, but this is probably a sign of a healthy debate rather than a weakness. More generally, those who take a religion-based ethical position will want to ask two questions about those ethical views that have no religious foundation.

First, are they capable not just of giving us certain ethical principles, but of motivating us to observe them? Though there have, of course, been good agnostics and good atheists, there can be no greater inspiration in principle than the thought that one's values are rooted in the being and purpose of God and are therefore written, as it were, into the very fabric of the universe; values, moreover, in response to which we find our fulfilment and blessedness.

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Second, given the fact that western civilisation has been so largely shaped by its Christian inheritance, are we living on our moral and spiritual capital? And, if so, how long can this last before it is all spent? The poet Stevie Smith loved the Anglican Church but had grave doubts, both philosophical and moral, about the truth of the Christian religion. Yet she also agonised about whether it was really possible to inspire people on the basis of a secular ethic. In her poem How Do You See , she writes: "I think if we do not learn quickly, and learn to teach children/ To be good without enchantment, without the help/ Of beautiful painted fairy stories pretending to be true/ Then I think it will be too much for us, the dishonesty/ And, armed as we are now, we shall kill everybody/ It will be too much for us, we shall kill everybody."

Richard Harries is bishop of Oxford.