"So, in practice, how would you ensure that a) all cases where the motives are not honest are avoided, and b) how we would continue this indefinitely, and c) why the current system doesn't achieve this?
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The current system does not achieve this because it does not properly allow for an individuals desires and wishes to be properly tested in advance of physician aided termination or withdrawal of care.
We are also talking about a highly specific set of circumstances here. The individuals concerned will have to make an application to a court or tribunal, on more than one occasion and over a period of time, to demonstrate true intent and lack of coercion.
In those cases, the circumstances surrounding the individuals desire to end their life will be fully explored, and explicit consent will have to be given - not just a living will or passive consent, or a family member passing on a supposedly expressed wish, or anything like that. What dishonest motive can you imagine that would effectively pass such a scrutiny? Any such system would seek a 2nd or 3rd informed medical opinion on both the prognosis and palliation. Full and frank discussion with a pyschologist / councillor to determine lucidity, state of mind and whether unseemly pressure is being applied. If you were a family member or friend wishing to bump someone off for material gain, unless you are trying to craft a best-seller or film, the very last thing you would wish to do would allow yourself to become part of such a detailed scrutiny.
So - we are talking about patients who are demonstrably compos mentis,fully conversant of their prognosis and up to date with what palliation might offer them who have explicitly expressed their wish as a free and uncoerced adult individual, and who have also explicitly given their consent - we are not talking about some elderly relative suffering dementia, or in a coma, or unable to participate in the process or anything like that.
The concern over an extension of the remit for state-sanctioned or physician- assisted dying is a genuine one, but it is not something that is an inevitable consequence. The Netherlands probably offers us the best example of the type of legal framework and assisted dying programme that I can think of. Such cases in the Netherlands are only sanctioned in cases of "hopeless and unbearable" suffering.
http://en.wikipedia.org/wiki/Euthanasia_in_the_Netherlands
You will note from the article that there is a lobby group in the Netherlands who are advocating the right to euthanasia for anyone over 70 who "feel tired of life". Now widening the remit of any state-sanctioned process to include such circumstances as being "tired of life" would be a step too far, and I seriously doubt they will be able to generate a sufficiency of support to effect any change in the law.