On December 21 an Italian doctor, Mario Riccio, disconnected a respirator that was keeping Piergiorgio Welby alive. Welby, who suffered from muscular dystrophy and was paralysed, had battled unsuccessfully in the Italian courts for the right to die. He said, "Thank you", three times to his wife, his friends and his doctor. Forty-five minutes later, he was dead.

His request to die led to heated debate in Italy, and it is unclear whether Riccio will be charged with any offence. At least one Italian politician has called for his arrest on a charge of murder.

Welby's death raises two questions - whether a person has a right to refuse life-sustaining medical treatment; and whether voluntary euthanasia is ethically defensible.

A patient's informed consent should be a prerequisite for all medical treatment, as long as the patient is a competent adult in a position to make a decision. Forcing medical treatment on such a patient who does not want it is tantamount to assault. We may think that the patient is making the wrong decision, but we should respect his or her right to make it. That right is recognised in most countries, but not, apparently, in Italy.

Even the Roman Catholic church has long held that there is no obligation to use "extraordinary" or "disproportionate" means to prolong life - a view reiterated in the Declaration on Euthanasia issued by the Sacred Congregation for the Doctrine of the Faith and approved by Pope John Paul II in 1980. That document states that to refuse burdensome medical treatment "is not the equivalent of suicide" but "should be considered an acceptance of the human condition, or a wish to avoid the application of a medical procedure disproportionate to the results that can be expected, or a desire not to impose excessive expense on the family or the community".

On that basis, Riccio was doing what anyone should have been prepared to do for Welby, who was unable to implement his refusal of burdensome medical treatment. So the case falls on the right side of the line drawn by Catholic doctrine, but does church doctrine draw the line in a sensible place? If an incurably ill patient can refuse burdensome treatment, knowing that this refusal will mean his or her death, why should an incurably ill patient who is not being kept alive by any medical treatment, but finds that the illness itself makes life burdensome, be unable to seek assistance in escaping that burden?

Defenders of Catholic teaching would answer that the latter patient intends to end his or her life, whereas the former patient merely intends to avoid the additional burden that treatment brings. Death is a foreseeable consequence of avoiding that burden, but it is a byproduct, not directly intended. If the patient could avoid the burden and yet continue to live, that would be his choice. Welby should not have been helped to die, they might argue, because he expressly said that he wanted to die, not that he wanted to avoid burdensome treatment.

This distinction is dubious. Both patients knowingly choose a course of action that will lead to death, rather than to a longer but burdensome life. By focusing on the intention to refuse burdensome treatment, rather than the broader implications of the choice, the church avoids the inhumane implication that patients must accept life-prolonging treatment, no matter how painful or costly it may be. But it does so at the cost of rendering incoherent its own vigorous opposition to assisted suicide and voluntary euthanasia.

Many countries recognise a legal right to refuse medical treatment. But only in the Netherlands, Belgium, Switzerland and the US state of Oregon are doctors allowed to assist a patient in ending his or her life by means other than withdrawing life-sustaining treatment.

The Netherlands, in particular, has been subjected to a relentless campaign of vilification. Critics allege that the legalisation of voluntary euthanasia has led to a breakdown of trust in the medical profession and all sorts of other dire consequences. But if these allegations are true, no one has told the Dutch. Despite a change of government in the Netherlands since voluntary euthanasia was legalised, no effort has been made to repeal the measure. There is simply no public support for such a move.

The Dutch know how voluntary euthanasia is practised in their country, they know that legal euthanasia has improved, rather than harmed, their medical care, and they want the possibility of assistance in dying. Isn't that a choice that everyone should have?

· Peter Singer is professor of bioethics at Princeton University; his books include Practical Ethics and Rethinking Life and Death www.project-syndicate.org