My mother is dying. Or at least, she is not truly living. She is in a twilight world, T.S.Eliot's “living and partly living”. Slowly but irrevocably she is losing every one of her faculties. It could take quite a while.

She is in what is called “end stage” Alzheimer's. She can no longer do anything for herself - get out of bed, dress, eat, walk, go to the lavatory. She can talk - but mostly it makes little sense. Does Bettina, now 87, want to be in this situation? Would she, to put it bluntly, prefer to die?

This week, the debate over assisted suicide has been refuelled both by the screening of the death of Craig Ewert, who suffered from motor neuron disease, at the Dignitas clinic in Switzerland, and the decision not to prosecute the parents of Daniel James, 23, who was paralysed after a rugby accident and who made the same journey.

Assisted suicide remains a crime in Britain. If, as campaigners hope, the law is changed, people with an incurable disease or whose life is severely circumscribed by an accident would be able to choose the time of their death. They could still be mentally alert but physically incapacitated. In my mother's case, it is the opposite: her every physical need is catered for. Her mental ability to manage her own body is disappearing, bit by bit.

Should she, too, along with sufferers of any other incurable illness, be able to choose to call it a day? Had she known that she would end up as she is now, would she - if permitted - have made a living will, stipulating that when no longer capable, she should be helped to die?

I believe not, and nor do I think that such a choice should be permitted.

I remember my mother as a loving, gentle and intelligent woman. Sent out to work at 14, she took her O and A levels in her fifties and won a university place. She did not take it up: by then she was enjoying success as a writer of short stories and features - the career she had never had when her children were small.

In her seventies she became forgetful. The process was so gradual that to close family members it was almost imperceptible. But there were clear signposts: the first time that she asked me who I was, or the holiday in Venice, where she praised the quality of the shopping at “Kingston upon Thames”. Conversation became more limited but the essential Betty was always there - the humour, the laughter, the singing. And there were flashes of insight. Staying with us on a weekend, she appeared in a mix of night and day garments, confessing that she was in a muddle. It was only through my father's steadfast support and care that she managed to keep going, her nights and days confused, increasingly nable to care for herself, shop or make meals - my father was once treated to kitchen paper spread with marmalade for his breakfast.

Both, though, were by then in their eighties. When he suffered a stroke, she had to go into temporary care. And she never came home. That was more than four years ago. She is a now a shadow of what she once was. But should the law permit the growing numbers of elderly with dementia to end their lives? There are some 700,000 people with degenerative diseases such as Alzheimer's in Britain. It has been predicted that there will be a million dementia sufferers by 2026.

Three months ago, Baroness Warnock, the government adviser and moral philosopher, said that there was “nothing wrong” with people being helped to die for the sake of their loved ones and society. She hoped that people would soon be “licensed to put others down” if they could not look after themselves and had become a burden. Those who were demented, she said, were “wasting people's lives - your family's lives - and ... the resources of the National Health Service”.

But who would make the decision for the demented person - or decide that the time was right? Would it be when they could no longer talk, or walk - or would it be now, for instance, when our family worries about whether there will be enough funds (draining out at more than £900 a week) for my mother's care?

Of course it is distressing to see someone you love slowly disintegrate. But I don't regard my mother as a burden. I don't want her to die and I believe that, even in her diminished state, she would not want that either. She can say little that is meaningful, but on occasion, when anxious or distressed, she has said that she does not want to die. In her Abbeyfield home in New Malden, Surrey, she is lucky enough to have superb and committed care. I feel that my mother is contented and peaceful in these final years. She can still gain pleasure from small things - a song, a cup of tea. She will smile, on a good day, to see me.

The current cry is all for dignity in death. And I would not deny mentally capable people the right to die at a time of their choosing. But it is dangerous territory for the mentally vulnerable. When death takes its toll by degrees, dignity in life is both achievable and worth achieving - even for the “living and partly living”.

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