I know a woman my own age who is so disabled by Parkinson's disease she is confined to a nursing home. She cannot walk, cannot speak and be comprehensible, cannot brush her own teeth, so jerky and uncontrollable are her movements - yet her mind is crystal clear.

I interviewed the journalist Jill Tweedie very shortly before she received her diagnosis of motor neurone disease. She, too, remained intellectually sound and therefore aware of her fate in the months it took for creeping paralysis to kill her.

I know another woman, much younger than I, who has been in a coma for seven years. I remember her as a lively child and engaging teenager before multiple sclerosis struck. She has parents, sisters, a husband and children. Shall I continue? I could.

Take each of these cases, multiply them by the thousands of others currently suffering and add all the sufferers to come. Place them, with their parents, children, lovers and friends on the scales of justice. On the opposite scale, put small, circular petri dishes which contain groups of dividing cells, none older than 14 days. They are part-animal/ part-human. Would you use the cells to save the people?

Put that way, is there a contest? If you value human life, if you have an ounce of compassion, surely your heart and mind will reach out to the suffering humanity. Could you, in all conscience, watch your fellow man robbed of life while still living, knowing that you might have a cure within grasp and failing to pursue it?

Those cures are the goal of the medical scientists at the forefront of stem-cell research. Clearly their motives are above reproach. The question facing the House of Commons (and the rest of us) is whether, in the passion of their pursuit, they have lost touch with the ethics that must govern it?

Let us look again at the scale with the petri dishes. (As with most contributors to this debate I here must declare a profound ignorance. I have no degree in neuroscience so everything I say is received information, though from sources I trust.) As I understand it, therefore, the petri dishes could contain chimeras (merged human and animal embryos), hybrids (a human egg fertilised with animal sperm or an animal egg fertilised with human sperm) or cytoplasmic hybrids referred to as "cybrids" (an animal egg with the nucleus removed, into which human cells are introduced).

Everything I have read so far suggests that for the most part it is cybrids - the least alarming of the hybrids - that will be used. However, if the bill is passed all three will be legalised.

I don't think it is only the religious element of the electorate that will experience a strong sense of unease at the thought. This legislation allows a breaching of the species divide. Unless you are a believer in the infallibility of scientists, surely that is a step which requires not just serious and informed debate - but a consensus. It is a subject even more contentious than the genetic modification of crops because once the principle is conceded the pressure of scientific advance will push the boundaries of it. We will all live with the consequences.

Let's leave religion out of it. What we all need to know is: what are the potential down sides and what are the side-effects?

I recall in the 1980s when Aids arrived, trailing fear of mass deaths (a fear that has been fulfilled in Africa if not here). There was talk of this unknown virus having jumped the species divide. Scientists linked HIV with chimpanzees. More recently we have had waves of hysteria about the prospect of a pandemic of bird flu - again with fears of species-jumping. And now we are contemplating legalising human/ animal hybrids which will produce stem cells that are 99.9% human and 0.1% cow. How much of a percentage is required to carry a virus?

Also, embryology is like devolution. It is a process not an event. Those debating the 1990 embryology bill deemed the cloning of animal-human cells to be abhorrent; unthinkable. The current bill in draft form outlawed it until scientists and patient groups successfully argued for change. So we should ask ourselves, what if the cell amalgams in the petri dishes prove a fruitful research pathway? What happens if hybrid cells are effective and would be even more effective at 28 days or three months or after implantation? Will they lead us, cell by dividing cell, to a future beyond our imaginings? Will our boundaries of what is acceptable expand until one day our children are looking into the eyes of a creature far and away more pitiable than those who now need the scientists' help? We have already seen a mouse with a human ear growing on its back - a sight which caused less of a stir than the finals of Strictly Come Dancing. The sphinx and the minotaur are still creatures of myth but for how long?

It is 20 years since I sat beside a scientist at dinner who told me about two-headed animals in a research centre in Scotland. Remember that science is, in its way, a religion and that it has its zealots. Research scientists, many convinced secularists, see the possibility of curing major diseases grow ever closer. They won't tolerate opposition, particularly when it emanates from a faith group that many of them regard as an anachronistic, superstitious and irrelevant.

But just as psychotherapists must remain in therapy themselves to prevent constant exposure to the unwell warping their thinking - and just as prime ministers must face the electorate to temper them from becoming power crazed - so scientists need a braking mechanism. They will say they have one in parliament. If the public is to be reassured by that, parliamentarians need an informed and comprehensive briefing. Equally importantly, they must have a free vote. And unless they are planning to list their faith group on their election manifestos, that must be free from the religious as well as the parliamentary whip.

I read that scientists have offered to meet MPs and bishops (and hopefully imams and rabbis) to explain the need for legislation. That is to the good. I am only surprised that specialist briefings for MPs haven't already taken place. As for the denial of a free vote: Gordon Brown's consistent propensity to paint himself into a corner must remain a matter of wonder. I never, however, forget the impact of personal experience on political action and Brown has a sick child.

And the upshot? My bet is that the research will continue, perhaps with more stringent checks and balances. The scientists deserve our gratitude for the help they hope to offer our bodies. The bishops deserve it for shouting a warning that we all do well to heed. The debate is being presented as having just these two sides, each claiming knowledge beyond our ken; the scientists through expertise, the bishops through faith. It silences the man and woman in the street. If we care about the world our children will inherit, it mustn't.