Debbie Purdy said she was ecstatic about her legal victory in the House of Lords, as well she might have been. The ruling represents a complete vindication for her in her long battle through the courts and means that the law on assisted suicide must be clarified. It also brings the whole subject of euthanasia back into the public arena.

In their final ruling before the summer recess and their re-emergence in the autumn as Supreme Court justices, the law lords said that the law as it stands was not as clear and precise as it should be. They instructed the Director of Public Prosecutions (DPP) to set out the factors he would take into account when deciding whether prosecutions should be brought. That meets Mrs Purdy's demand that those contemplating assisted suicide, and those who would help them, should have a better idea of where they stand in relation to the law.

Mrs Purdy also won on a second point, when the Law Lords agreed, citing the European Convention on Human Rights, that she had the right to choose how she died. The central purpose of her fight had been to ensure that her husband, Omar Puente, would not face prosecution if he helped her to end her life abroad. She seems to have achieved this, and quite a bit more.

The ruling is significant not just for what it said. It also crowned an inspiring personal campaign. Rebuffed in two lower courts, Mrs Purdy – who suffers from multiple sclerosis – insisted on fighting on. Her victory demonstrates that one determined individual, well-informed and passionate about a cause, has a fair chance against what often seems an impenetrable wall of political and judicial authority. That says something good about our country, its institutions and its courts. Where others went before and lost – notably the late Diane Pretty, who strove for the right to an assisted suicide in Britain – Mrs Purdy, with a narrower agenda, has prevailed.

For all the joy and relief in the air around Westminster yesterday, however, there will be those who do not share Mrs Purdy's elation, and we admit to misgivings of our own. We would particularly question her description of the ruling as "a huge step towards a more compassionate law". In principle, clarity in the law is highly desirable, even essential. But there are instances – and difficult ethical issues are among them – where exactitude can be more of a hindrance than a help.

The DPP has now to identify considerations that will militate for or against prosecution in cases of assisted suicide. Spelling these out could turn out to be limiting rather than liberating. It risks giving rise to perverse judgments that match the letter rather than the spirit of the law and invite reversal on appeal. The wise exercise of discretion has a role.

The reality is that so far no one in the UK has been prosecuted for accompanying someone to commit suicide abroad. But it is not impossible to conceive of situations in which the motives of friends or relatives might be neither noble nor compassionate – to use Mrs Purdy's words. There are dangers in reversing the presumption that helping to end someone else's life prematurely is a bad thing. Not to be ignored either is the negative message any change in the law could send to those with terminal or progressive illnesses.

As the population ages and a new generation increasingly expects to have control over death as well as life, these questions need to be debated. Debbie Purdy's victory must be the beginning of such a discussion, not its end.