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My understanding is that natural law/ spontaneous human morality apparently accepts the action of 'letting die', under certain circumstances; and the qualitative distinction between letting die and actively killing.
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From my reading of the anthropology of hunter gatherer tribes, it seems very likely that passive infanticide of newborns by a mother abandoning her baby, and also abandonment of chronically incapable elderly relatives, are regarded as morally acceptable actions under some circumstances
(although certainly very regrettable and an occasion of grief and mourning which may be intense and prolonged).
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Indeed, as convincingly argued by Sarah Blaffer Hrdy in Mother Nature; infanticide by neglect or abandonment is probably the fall-back method by which humans (as a species) control the number of offspring (when the normal method of spacing-out offspring by the contraceptive effect of lactation has failed).
(Other mammals fail to conceive under stress, reabsorb the fetus while it is still in the womb and have other methods of controlling the spacing of offspring according to circumstances.)
The abandonment (leaving-behind) of chronically-incapable elderly relatives seems to be the norm among hunter gatherers and nomadic herders.
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It also seems that these actions of passive abandoning and letting die do not elicit a spontaneous and almost-universal abhorrence among mankind in general - in the way that active killing of newborn infants or incapable elderly would elicit a spontaneous and almost-universal abhorrence.
(Which is not to say that this abhorrence cannot be overcome - it can be overcome. Humans can, under certain circumstances, actively kill infants and elderly relatives and can regard this as morally justified. Nonetheless, there is a spontaneous abhorrence of these actions.)
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In practice, the abandonment of infants and elders means (so far as we can tell) all-but-certain death; and quite likely the horrible death of being eaten, perhaps while still alive, by predators or scavengers.
(But this fate would not be known for sure; and there may well be a hope of some fortuitous rescue or supernatural intervention, and the hope that this had in fact happened.)
This suggests that - for our ancestors, and probably spontaneously for all humans - the 'mercy killing' of infants and elderly relatives was probably perceived as being morally worse than allowing horrific suffering.
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Note that I am stating this as a factual observation, and not in terms of the 'naturalistic fallacy' of 'what is, is right'.
But I think these facts need to form the basis of honest moral discussion.
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I regard it as simply false to assume that abandoning and letting die of of the newborn and incapable elderly is something that humans, qua humans, find morally abhorrent.
And the prohibition of these acts of letting die or passive killing is more or less specific to Christianity. It was, indeed, one of the distinguishing marks that set apart the early Christians from those who surrounded them.
If, then, it is to be argued (perhaps, although not necessarily, by Christians) that humans ought not to commit passive infanticide or 'euthanasia' of the elderly by stopping active interventions etc; then the argument cannot (in my opinion) be based on natural morality, nor can it depend on a spontaneous abhorrence of humans qua humans for these actions.
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It is also false to argue that letting die amounts to the same thing (morally speaking) as active killing; since spontaneous human morality recognizes a qualitative difference between these actions.
To abandon someone to almost-certain death is not the same as murdering them - according to natural law.
Spontaneous human morality says that neglect of a person even unto their death is not the same as purposive destruction of life.
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Humans are not 'naturally' inclined to regard the prevention of suffering of loved ones as a higher moral imperative than the avoidance of oneself killing loved ones.
(An exception occurs during mental illness - specifically melancholia, when it is fairly common for a profoundly depressed parent to kill their family, then kill themselves, in order to protect the family from what is perceived as an unendurably miserable world.)
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If, then, an argument is to be mounted that (probably-) fatal abandonment of infants and incapable elderly relatives is morally wrong, then the reasons for this prohibition must properly be based upon Christian revelation; and not on natural law.
Following from this, there is no coherent, truthful and rational argument by which non-Christians can prohibit the passive letting die of (for example) infants and elderly relatives.
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(Non-Christians might, nonetheless, wish to prohibit these actions; what I am saying is that non-Christians would have no coherent, truthful and rational arguments by which to justify such prohibitions.)
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