Home secretary Jack Straw's announcement on severe personality disorders is rightly attracting much attention. His objective is to create a safer society by identifying and detaining severe psychopaths. It is controversial because it proposes preventive detention.

The proposal's origin is understandable, and its intentions laudable in principle. The awful crimes committed by Michael Stone, who was refused admission to a psychiatric hospital because he was deemed untreatable, may seem preventable in retrospect. Equally, the release from prison of sex offenders such as Robert Oliver, posing a self-acknowledged risk to society, is frightening and unacceptable.

At the root of the proposal lies the conviction that psychopaths - more properly, people suffering from anti-social personality disorder - are a homogeneous and uniquely dangerous group separable from innocent, law- abiding people, common criminals and those with a real mental illness. This would, if so wished, allow early identification, detention and prevention of their yet-to-be committed crimes.

The idea of a separate identity is already controversial, since psychopaths are a sub-group both of criminals - about 50 per cent of men in prisons have been identified as psychopaths - and of mentally ill people. Psychiatry does not offer much enlightenment. Personality disorder in general is defined as an enduring pattern of inner experience and behaviour that deviates markedly from the expectations of the individual's culture. This is a definition riddled with ambiguity.

The widely-used diagnostic classification system DSM IV gives seven criteria for anti-social personality disorder, most involving criminal acts, but not necessarily of a severe nature. The term 'severe personality disorder' is not separately recognised. It is questionable whether we are able to diagnose people preventively, since the diagnosis of anti-social personality disorder is based on past nasty behaviour.

Of most concern to psychiatrists is unstable presentation. For example, Michael Stone heard voices in the past. It is not at all unusual for a diagnosis to change over time from schizophrenia to mood disorder and to personality disorder, in any direction and more than once, often related to treatment response. The concept of personality disorder seems neither stable nor reliable, and possibly not even valid. Should someone be picked out for life-long detention during their psychopathic middle period because they did not respond to medication at the time?

Treatability, or rather its absence, has emerged as a crucial but rather confused concept in the debate. Part of this confusion is about the treatability per se of psychopaths, part about its association with the Mental Health Act. As with so many aspects of personality disorder, no consensus exists about the effectiveness of therapy, even among psychiatrists. This is unsurprising since it is unclear how treatability is determined. It certainly includes more than medication and psychotherapy, but how much more? If cure is the aim, which is often implied in debates about personality disorder, expectations should be very low indeed.

But why should cure be the criterion? It isn't for people with schizophrenia, whose problems do not always respond to treatment, but who are not excluded from care on these grounds.

However, the Mental Health Act offers a very different view, justifying detention because treatment is likely to alleviate or prevent a deterioration of the condition. This is so broad that having a nice warm bath might be sufficient. It is difficult not to have some sympathy with politicians trying to grasp such slippery concepts, which always seem to take on shapes convenient to support the arguments of mental health workers and, of course, libertarians.

Exposing the intellectual failure within the Mental Health Act does not help. If it is right to detain people who have severe personality disorders but are treatable, why on earth is it wrong to detain people who are regarded as untreatable, but who continue to pose exactly the same or a worse risk to the public?

This is not grasping that the Mental Health Act is a major infringement of civil rights, only permissible on paternalistic principles, because an individual needs treatment and failure to respond to this need would be harmful.

Moving from ends to means, some safeguards in the process of detention are proposed, although no details have been given yet. I suspect that psychiatrists and/or the police will be expected to submit cases to a court. The difficulty of predicting violence in the first place is so well known that I will not waste space on it.

Very important, though, is what level of risk will be deemed unacceptable to society, and whether this will be on the balance of probabilities - handing frightening powers to the court - or, beyond reasonable doubt, making action near impossible. One worrying suggestion is that standard of proof would be similar to that within the Mental Health Act, which serves a very different purpose. We have to remain alert to the difference between treatment and detention.

Once on the inside, it is expecting a lot of semi-judicial reviews to judge the risk of criminal acts reliably and objectively, especially since behaviour has to be generalised from small units to the wider community. It will need brave people to run such a system, since society will be outraged whenever a resident is released, or - imagine - commits an offence.

The whole idea does not stand up to analysis, whether from the perspective of human rights, psychiatry or practicalities. It is also unnecessary, since reviewable sentencing - such as proposed in the Fallon inquiry into Ashworth Hospital - or extended supervision could deal with most cases.

The proposed legislation is undoubtedly inspired by true sentiment and understandable frustration concerning exceptionally unpleasant individuals, but this is not the basis for good law. Better drop it now.