When I was a young boy, playing on the floor of my grandmother's front room, I glanced up at the television and saw Death, talking to a knight. I didn't know much about death at that point. It was the thing that happened to ­budgerigars and hamsters. But it was Death, with a scythe and an amiable manner. I didn't know it at the time, of course, but I had just watched a clip from Ingmar Bergman's The Seventh Seal, wherein the knight engages in protracted dialogue, and of course the ­famous chess game, with the Grim Reaper who, it seemed to me, did not seem so terribly grim.

The image has remained with me ever since and Death as a character ­appeared in the first of my Discworld novels. He has evolved in the series to be one of its most popular characters; implacable, because that is his job, he appears to have some sneaking regard and compassion for a race of creatures which are to him as ephemeral as mayflies, but which nevertheless spend their brief lives making rules for the universe and counting the stars.

I have no clear recollection of the death of my grandparents, but my ­paternal grandfather died in the ambulance on the way to hospital after just having cooked and eaten his own dinner at the age of 96. He had felt very odd, got a neighbour to ring for the doctor and stepped tidily into the ambulance and out of the world. A good death if ever there was one. Except that, ­according to my father, he did ­complain to the ambulance men that he hadn't had time to finish his pudding. I am not at all sure about the truth of this, because my father had a finely tuned sense of humour that he was good enough to bequeath to me, presumably to make up for the weak bladder, short stature and male pattern baldness which regrettably came with the package.

My father's own death was more protracted. He had a year's warning. It was pancreatic cancer. Technology kept him alive, at home and in a state of reasonable comfort and cheerfulness, for that year, during which we had those conversations that you have with a dying parent. Perhaps it is when you truly get to know them, when you ­realise that it is now you marching ­towards the sound of the guns and you are ready to listen to the advice and reminiscences that life was too crowded for up to that point. He ­unloaded all the anecdotes that I had heard before, about his time in India during the war, and came up with a few more that I had never heard. Then, at one point, he suddenly looked up and said, "I can feel the sun of India on my face", and his face did light up rather magically, brighter and happier than I had seen it at any time in the previous year, and if there had been any justice or even narrative sensibility in the ­universe, he would have died there and then, shading his eyes from the sun of Karachi.

He did not.

On the day he was diagnosed my ­father told me, "If you ever see me in a hospital bed, full of tubes and pipes and no good to anybody, tell them to switch me off." In fact, it took something under a fortnight in the hospice for him to die as a kind of collateral damage in the war between his cancer and the morphine. And in that time he stopped being him and started becoming a corpse, albeit one that moved ever so slightly from time to time.

On the way back home after my ­father's death I scraped my Jag along a stone wall in Hay-on-Wye. To be fair, it's almost impossible not to scrape Jags along the walls in Hay-on-Wye even if your eyes aren't clouded with tears, but what I didn't know at the time, but strongly suspect now, was that also playing a part in that little ­accident was my own disease, subtly making its presence felt.

When the specialist gave me the news that I had posterior cortical ­atrophy, a rare form of Alzheimer's ­disease, I quite genuinely saw him ­outlined in a rectangle of flaming red lines. The whole world had changed.

PCA manifests itself through sight problems, and difficulty with topological tasks, such as buttoning up a shirt. I have the opposite of a superpower; sometimes I cannot see what is there. I see the teacup with my eyes, but my brain refuses to send me the teacup message. It's very Zen. First, there is no teacup and then, because I know there is a teacup, the teacup will appear the next time I look. I have little work-arounds to deal with this sort of thing – people with PCA live in a world of work-arounds.

If you did not know there was ­anything wrong with me, you would not know there is anything wrong with me. The disease moves slowly, but you know it's there. ­Imagine a very, very slow-motion car crash. Nothing much seems to be ­happening. There's an occasional little bang, a crunch, a screw pops out and spins across the dashboard as if we're in Apollo 13. But the radio is still playing, the heater is on and it doesn't seem all that bad, except for the certain knowledge that sooner or later you will definitely be going headfirst through the windscreen.

I have heard it said that some people feel they are being avoided once the news gets around that they have Alzheimer's. For me it has been just the reverse. People want to talk to me – on city streets, in theatre queues, on aeroplanes over the Atlantic, even on country walks. They want to tell me about their mother, their husband, their grandmother. Increasingly, they want to talk about what I prefer to call "assisted death", but which is still called, wrongly in my opinion, ­"assisted suicide".

As a pallid and nervous young ­journalist I got to know about suicide. It was part of my regular tasks to sit in at the coroner's court, where I learned all the manifold ways the disturbed ­human brain can devise to die. Newspapers were a little more kindly in those days, and we tended not to go into too much detail, but I had to listen to it. And I remember that coroners never used the word "insanity". They preferred the more compassionate ­verdict that the subject had "taken his life while the balance of his mind was disturbed". There was ambivalence to the phrase, a suggestion of the winds of fate and overwhelming circumstance.

In fact, by now, I have reached the ­conclusion that a person may make a decision to die because the balance of their mind is level, realistic, pragmatic, stoic and sharp. And that is why I dislike the term "assisted suicide" applied to the carefully thought-out and weighed-up process of having one's life ended by gentle medical means.

The people who thus far have made the harrowing trip to Dignitas in Switzerland to die seemed to me to be very firm and methodical of purpose, with a clear prima-face case for wanting their death to be on their own terms. In short, their minds may well be in better balance than the world around them.

And once again I remember my ­father. He did not want to die a curious kind of living death. He wasn't that kind of person. He wanted to say ­goodbye to me, and knowing him, he would probably have finished with a joke of some sort. And if the nurses had put the relevant syringe in the ­cannula, I would have pressed it, and felt it was my duty. There would have been tears, of course there would; tears would be appropriate and insuppressible.

I got involved in the debate surrounding "assisted death" by accident after taking a long and informed look at my future as someone with Alzheimer's and subsequently writing an article about my conclusions. As a result of my "coming out" about the disease I now have contacts in medical research industries all over the world, and I have no reason to believe that a "cure" is imminent. I do think, on their good advice, that there may be some very ­interesting developments in the next couple of years and I'm not the only one to hope for some kind of stepping stone – a treatment that will keep me going long enough for a better treatment to be developed.

Back in my early reporting days I was told that nobody has to do what the doctor tells them. I learned this when chief reporter George Topley slung my copy back at me and said, "Never say that a patient has been ­released from hospital unless you are talking about someone who is being detained on mental grounds. The proper word is 'discharged', and even though the staff would like you to ­believe that you just can't walk out ­until they say so, you damn well can. Although, generally speaking, it's best not to be dragging a portable life support system down the steps with you." George was a remarkable journalist who as a fiery young man would have fought fascism in the Spanish civil war were it not for the fact that he stowed away on the wrong boat and ended up in Hull.

I remember what George said and vowed that rather than let Alzheimer's take me, I would take it. I would live my life as ever to the full and die, ­before the disease mounted its last ­attack, in my own home, in a chair on the lawn, with a brandy in my hand to wash down whatever modern ­version of the "Brompton cocktail" some ­helpful medic could supply. And with ­Thomas Tallis on my iPod, I would shake hands with Death.

This seems to me quite a ­reasonable and sensible ­decision for someone with a serious, incurable and ­debilitating disease to elect for a medically assisted death by appointment. These days, non-traumatic death – deaths that don't, for example, involve several cars, a tanker and a patch of ice on the M4 – largely take place in hospitals and ­hospices. Not so long ago, they took place in your own bed. The Victorians knew how to die. They saw a lot of death. And Victorian and Edwardian London were awash with what we would call recreational drugs, which were seen as a boon and a blessing to all. Departing on schedule with the help of a friendly doctor was quite usual.

Does that still apply? It would seem so. Did the Victorians fear death? As Death says in one of my own books, most men don't fear death, they fear those things – the knife, the shipwreck, the illness, the bomb – that proceed, by micro seconds if you're lucky and many years if you're not, the moment of death.

And this brings us into the whole care or killing argument.

The Care Not Killing Alliance assures us that no one need consider a voluntary death of any sort since care is always available. This is questionable. Medicine is keeping more and more people alive, all requiring more and more care. Alzheimer's and other ­dementias place a huge care burden on the country, a burden that falls initially on the next of kin, who may even be elderly and, indeed, be in need of some sort of care themselves.

A major objection frequently flourished by opponents of assisted dying is that elderly people might be illegally persuaded into "asking" for assisted death. Could be, but the Journal of Medical Ethics reported in 2007 that there was no evidence of the abuse of vulnerable patients in Oregon, where assisted dying is currently legal. I don't see why things should be any different here.

Last year, the government finally published guidelines on dealing with assisted death. They did not appear to satisfy anybody. It seems that those wishing to assist a friend or relative to die would have to meet quite a large number of criteria in order to escape the chance of prosecution for murder. We should be thankful that there is, in theory, some ­possibility that they might not be ­prosecuted but, as laid out, the best anyone can do is keep within the rules and hope for the best.

That is why I and others have ­suggested some kind of strictly non-­aggressive tribunal that would establish the facts of the case well before the ­assisted death takes place. This might make some people, including me, a little uneasy as it suggests the govern­ment has the power to tell you whether you can live or die. But, that said, the government cannot sidestep the ­responsibility to ensure the protection of the vulnerable and we must respect that. It grieves me that those against assisted death seem to assume, as a matter of course, that those of us who support it have not thought long and hard about this very issue. It is, in fact, at the soul and centre of my argument.

The members of the tribunal would be acting for the good of society as well as that of the applicant – horrible word – to ensure they are of sound and informed mind, firm in their purpose, suffering from a life-threatening and incurable disease and not under the ­influence of a third party. It would need wiser heads than mine, though heaven knows they should be easy enough to find, to determine how such tribunals are constituted. But I would suggest there should be a lawyer, one with ­expertise in dynastic family affairs who has become good at recognising what somebody really means and indeed, if there is outside pressure. And a ­medical practitioner experienced in dealing with the complexities of serious long-term illnesses.

I would also suggest that all those on the tribunal are over 45, by which time they may have acquired the rare gift of wisdom, because wisdom and compassion should, in this tribunal, stand side-by-side with the law. The tribunal would also have to be a check on those seeking death for reasons that reasonable people may consider trivial or transient distress. I dare say that quite a few people have contemplated death for reasons that much later seemed to them to be quite minor. If we are to live in a world where a ­socially acceptable "early death" can be allowed, it must be allowed as a ­result of careful consideration.

Let us consider me as a test case. As I have said, I would like to die peacefully with Thomas Tallis on my iPod before the disease takes me over and I hope that will not be for quite some time to come, because if I knew that I could die at any time I wanted, then suddenly every day would be as ­precious as a million pounds. If I knew that I could die, I would live. My life, my death, my choice.

This is an edited extract of Terry Pratchett's Richard Dimbleby lecture, Shaking Hands With Death, which was broadcast on BBC1 on 1 February