Legalizing voluntary euthanasia would lead to a creeping acceptance of non-voluntary euthanasia in cases, where a patient cannot make a request for the termination of his life (the elderly with dementia, unconscious patients, children).
One of the most controversial issues surrounding the legal euthanasia is safeguarding against a possible abuse of the practice. Opponents of the legal euthanasia claim, that it's not possible to create a set of guidelines, which couldn't be interpreted in a way, that would let doctors perform non-voluntary or sometimes even involuntary euthanasia. Furthermore, opponents of the legal euthanasia argue, that by accepting premises of voluntary euthanasia, that is there are lives not "worth" living, one cannot stop a "slide" towards acceptance of non-voluntary euthanasia, since the premises for latter apply as well to former.
As shown by the example of the Netherlands' euthanasia guidelines and its practical implementation, it is impossible to create such a set of guidelines, which wouldn't be vague and open to interpretation by physicians and/or patients. The sole concept of "unbearable pain" mentioned verbatim in Dutch guidelines cannot be legally defined, so it's impossible to create an objective, precise and strict list of conditions that have to be fulfilled for a voluntary euthanasia to occur. In light of this situation and possible interpretations made by practicing physicians and their patients, a slow, gradual process of widening the scope of the legal euthanasia is ultimately inevitable since the underlying assumption - that there are objective circumstances that make ending of a one's life preferable and ethically more justified than prolonging it - can be used to argue, that non-voluntary euthanasia is ethically permissible.
According to various researchers, there is no proof that legalizing euthanasia leads to an acceptance of a non-voluntary euthanasia. The guidelines set by the Dutch government, even if vague, work as intended and there is no definitive proof, that the scope of a practice in question was in time widened. Active, non-voluntary euthanasia remains illegal under all jurisdictions, even in those, where the active, voluntary euthanasia was legalized.
There has to be a set of clear, precise rules to limit the scope of the legal euthanasia so it will not be abused. Creating such set of rules is impossible, since there is no objective benchmark for a sensation of pain. Therefore, since we already accept the premise, that there are situations where is preferable to end a life instead of prolonging it, the slide towards the acceptance of non-voluntary euthanasia is inevitable.
The slippery slope argument might be a logical fallacy unless there is a high probability of consequences mentioned in an argument. No legalization of non-voluntary euthanasia ever occurred, thus, in this context, the argument in question is a logical fallacy.
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