In Ted Honderich (ed.), The Oxford Companion to Philosophy, Oxford, 1995, pp.445-446
Presumably no society could survive unless it had some restrictions on its members killing each other. But the prohibitions that societies have on killing vary greatly. In Greek and Roman times to be a human - that is, a member of the species Homo sapiens - was not sufficient to guarantee that one's life would be protected. Slaves or other 'barbarians' could be killed, under conditions that varied from time to time; and deformed infants were exposed to the elements on a hilltop. The coming of Christianity brought a new insistence on the wrongness of killing all born of human parents, in part because all humans were seen as having an immortal soul, and in part because to kill a human being is to usurp God's right to decide when we shall live and when we shall die. Non-human animals, on the other hand, remained unprotected because they were believed to have been placed by God under man's dominion. This doctrine of the sanctity of all (and only) human life remains the orthodox view on the morality of killing.
Some contemporary philosophers, among them Jonathan Glover, James Rachels, and Peter Singer, have challenged this orthodoxy, arguing that membership of a given species - for example, Homo sapiens - cannot in itself determine the value of a being's life, or the wrongness of killing that being. Rather, this wrongness must depend on some morally relevant characteristics that the being has. Sentience, or the capacity to feel pleasure or pain, seems to be a minimal characteristic, and so the killing of plants is not wrong in itself. In addition to sentience, however, Glover gives an important place to the being's capacity for autonomy, for making his or her own decisions (including a decision about whether or not to continue living). Killing an autonomous being against that being's will is the most drastic possible violation of autonomy, and this makes it more seriously wrong than the killing of a sentient being not capable of autonomy.
Rachels focuses on whether the being can live a biographical, rather than a merely biological, life, which is similar to the emphasis given by Singer to the ability to see oneself as having a past and a future. To kill such a being, unless at the being's request, thwarts the preferences for the future that the being may have, and this makes the killing wrong in a way that is additional to any wrong that may be incurred by the killing of a sentient being unable to form any preferences for the future.
The effect of these arguments is to distinguish a class of beings whom it is especially wrong to kill. The term 'person' is often used to distinguish this class from the class of human beings as a whole, for not all human beings are autonomous, or capable of seeing themselves as having a past and a future. Infants, and the profoundly intellectually disabled, for example, are not. Chimpanzees, on the other hand, appear to be persons in this sense. Hence it is an implication of this view that, other things being equal, it is worse to kill a normal chimpanzee than a profoundly intellectually disabled human being. Of course, to arrive at a final judgement about the wrongness of killing any being, we need to consider also the effect of the killing on relatives and friends, and on the community as a whole.
The slippery slope argument is often used as an objection to any change in our attitude to killing human beings. We should, however, be equally aware of the possible undesirable effects of, for example, allowing severely disabled infants to die slowly from dehydration or infection because we believe it wrong to kill them.
Jonathan Glover, Causing Death and Saving Lives (Harmondsworth, 1977).
James Rachels, The End of Life (Oxford, 1986).
Peter Singer, Practical Ethics, 2nd edn. (Cambridge, 1993), chs. 4-6.