Considering that all men are mortal, we are curiously unwilling to acknowledge that death, our inevitable fate, should not always be postponed. The Romans recognised that suicide was sometimes not only an admirable but a rational end to life. I think they were right and that there are situations of suffering where to kill oneself is the most reasonable and the most desirable course to take. But committing suicide is difficult, especially if one is ill and under constant surveillance. And though suicide is not illegal, as the law stands, one cannot ask help from anyone else, because assisting suicide is a criminal offence.

On Friday, Lord Joffe's bill to permit assisted dying for the terminally ill is to receive its second reading in the House of Lords. It is a bill of limited scope, allowing doctors to provide patients with the means to commit suicide, but only those who ask repeatedly for help to die, who are competent to make their own decisions and who are suffering acutely in the terminal stage of an incurable illness.

Lord Joffe introduced a different bill in 2004, which sought to legalise both assisted suicide and euthanasia for those who were terminally ill and wanted to die. That bill was debated, but not voted on in the House of Lords. The new bill is narrower and is closely modelled on the state of Oregon's Death with Dignity Act 1997, which permits only assisted suicide, not euthanasia.

One of the arguments against Lord Joffe's bill has been that it will distract attention from the need for better funding for palliative care, by seeming to offer suicide as an alternative. However, the experience of the state of Oregon does not bear this out: palliative care is better and more widely available there than almost anywhere in the US. We shall doubtless hear this argument on Friday, but it is not strong. No one seriously supposes that assisted suicide should be a replacement for palliative care, which is in urgent need of funding and of better trained doctors and nurses.

If palliative care were perfect, the dying might no longer need to ask for assisted suicide and the bill, if it became law, might remain unused. But this is a distant prospect and I do not believe that everyone would prefer palliative care. There are those for whom it would be a nightmare and who would prefer death to the drawn-out process of being kept alive and conscious, however kind, attentive and competent their carers. Such people may already have exercised their legal right to refuse all further treatment, but may yet linger in their journey towards death.

We must remember the case of poor Diane Pretty, who suffered from motor neurone disease and whose plea for a right to assisted suicide was rejected by the courts both here and at Strasbourg in 2002. She died as she had feared, the only possible palliation of her agony being sedation so powerful that she was virtually unconscious. We were told that she 'died peacefully'. I suppose unconsciousness is peaceful enough, but death when she asked for it might have been better.

So what are the other objections to the bill? Most of those who oppose it do so on more grounds than one and so the arguments become difficult to disentangle. The most abstract objection is based on the sanctity of human life. This argument has weight, when proposed by those for whom it is part of their religious faith. The Office of the Chief Rabbi, for example, in written evidence to the select committee which examined the earlier version of Lord Joffe's bill, said: 'Jewish tradition views life as a precious gift from God, not something we can dispose of at will.'

Many Christians and Muslims would echo his words. However, especially for those who are more inclined to think of life as lived by a particular person, the person valued rather than the life itself, it is possible to question whether the sanctity of life is a principle from which parliament can properly derive its decisions. Legislation must be for everyone, whether they possess religious faith or not, and legislators must therefore seek what is likely to be most conducive to the common good, regardless of faith. Here the arguments become empirical and should, if possible, be supported by evidence.

By far the greatest number of those who oppose the bill do so on the grounds that it forms the top of a slippery slope down which we shall all slide if assisted suicide is permitted. The slope has many tracks leading inevitably, it is supposed, to different kinds of disaster at their foot. Although the slippery-slope argument is necessarily speculative, dealing as it does with a hypothetical future, those who use it often cite analogies from other laws that have become gradually less restrictive - the abortion law in this country, for example, or the law governing euthanasia in the Netherlands.

And, in a very general sense, it is doubtless true that one thing leads to another and vigilance would be needed to ensure that the safeguards in the bill, such as the requirement that the patient be fully competent, were not relaxed without further legislation and that doctors did not become careless of the law.

For, as it stands, the bill is deliberately hedged with safeguards and would have strictly limited application. These restrictions block our descent down the slope. And so one common version of the slippery-slope argument, that if the bill were enacted, those with disabilities, including mental disabilities, would come to be killed off indiscriminately is actually irrelevant. For to be helped to die, a patient has to have asked repeatedly for assistance and has to be shown to be of sound mind. Indeed, if anything, it could be suggested that the bill would offer more protection to the vulnerable than they have at present. For, as things are, a doctor may administer drugs to relieve the suffering of the terminally ill, even if these drugs may shorten life and this without any request from the patient.

The appeal to so-called 'double effect', or unintended consequences, may be used to cover many cases of 'easing the passing'. At least it could be argued that the proposals in Lord Joffe's bill would make clear what was and what was not permissible treatment at the end of life. I do not think that this is a very strong argument in favour of the bill. If doctors are minded to hide behind the belief that you can claim responsibility for your good intentions, and disclaim it for the harmful consequences that are likely to follow, they will continue to do so. And I would admit that many patients benefit from this strange belief at the end of their lives. But this has nothing to do with assisted suicide.

Doctors and nurses have their own objections to the bill. They argue that their professional training demands they seek to preserve life, not to end it and, therefore, they are unable to bring themselves to be the immediate cause of a patient's death. Moreover, if they helped someone to commit suicide, this would lead to the end of all trust between patients and doctors. It is this argument above all others that determines me to support the bill.

Patients who are facing death and know that they are, and who are in great pain or distress, want nothing except that things should be less intolerable. It is for this that they should be able to trust their doctors. Death is in sight for them; accepting it, they may want it sooner rather than later. If this is their case, then it seems to me both unreasonable and dictatorial that they are not to have their own rational wish granted, but are to be subjected to the ethos of the medical profession instead. It is an ethos apparently unable to accept the choice of death. And yet, of all people, doctors should not be afraid of death.

With the advances in the technology of keeping people alive, it is time to think afresh about what it means to die at the proper time. Lord Joffe's bill is a step, though a small one, in this direction.

· Baroness Mary Warnock is a crossbench peer and author of 'Nature and Morality'