Science tells us what it is possible to do but society must decide what is the right thing to do. That is why it is up to MPs to decide which parts of the government's Human Fertilisation and Embryology Bill should reach the statute book. There have been massive scientific advances since the current legislation governing this area became law in 1990. Rationalists argue that the bill is merely a catching up exercise, but it is not that simple. The science involved is so advanced and the average level of scientific understanding in Britain so limited that it is difficult to have an informed debate. Additionally, this area comes up against some pretty basic philosophical questions about what it means to be human and whether there are ethical limits in the quest to cure disease.

Passion polarises the debate and sometimes ignorance clouds it. These are not issues that divide politicians or the public along traditional party lines and it is right that the Labour government has followed opposition parties in allowing a free vote on several controversial provisions in the bill, including hybrid embryos and what are known as "saviour siblings".

In January British scientists obtained permission for two projects using hybrid, or "admixed", embryos. The legislation would merely give explicit parliamentary approval. Scientists say they need them to study how genetic defects develop, causing diseases such as Parkinson's. Ultimately, stem cells formed in this process may be used to cure them. The alternative would be to use donated human eggs, but there is a drastic shortage. The word hybrid conjures up chimeric monsters, and some religious opponents of embryo research have recognised the instinctive unease felt by many.

Scotland's Roman Catholic leader, Cardinal Keith O'Brien, went further by attacking the bill as opening the door to experiments of "Frankenstein proportion". Yet among the public, and even within the Christian community, opinions are divided between those who find such research morally repugnant and those who feel a moral duty to help people dying of degenerative diseases. Defenders of the legislation point to the stipulation that all research embryos must be destroyed within 14 days and never reimplanted. As the Bishop of Oxford has suggested, for those who fear science is sliding down a slippery slope, this is an important rail to hold on to. We should be proud of Britain's lead in this research area but it must always be subjected to strict regulation.

Saviour siblings, born to provide a tissue match for a brother or sister with a serious genetic condition, are equally controversial. There have been six to date in Britain and they should remain rarities. Babies are gifts, not products, and compassion for a sick child must always be balanced by concern for the psychological impact on the sibling. There should also be a distinction between using blood from an otherwise discarded umbilical cord and expecting a saviour sibling to provide what amounts to spare parts for a brother or sister. Finally, these scientific lifelines must never be stretched to include non-life-threatening conditions because that risks opening the door on designer babies. Proper understanding and intelligent debate are essential because public acceptability has an important role in applying the brake to scientific possibility.