I propose that the moral significance of the suffering, mutilation, and death of non-human animals rests on the following, which may be called the overflow principle: Act towards that which, while not itself a person, is closely associated with personhood in a way coherent with an attitude of respect for persons. So stated, the overflow principle is intended to express a strict requirement of morality, although the principle will no doubt have ramifications within the aspirational dimension of morality as well.
One might argue for the overflow principle in a rule-consequentialist fashion, arguing that the teaching of such a principle will be ultimately conducive to the happiness of persons. But equally, the overflow principle might be made plausible by being exhibited as part of a way of life having respect for persons at its centre. In any case, the overflow principle would seem to be as well ensconced in the moral consciousness of the plain man as, say, the principle that gratitude is due to benefactors.
One application of the overflow principle is the principle of respect for the dead. Although a dead body is not a person, still the fact that it (so to speak) was a person means that it ought not to be treated like ordinary garbage. Alternatively, we may say that respect for persons overflows to the human body, which forms the visible aspect of the bulk of the persons with whom we are acquainted, and which persists when the person ceases to exist in death. Another and more controversial application is that human sexuality, since it is concerned with the generation of new persons, has a moral significance greater than that possessed by, say, pinball. Yet another application is that members of the human species who are not persons, even by virtue of their potentiality, still ought to be treated, in some respects at least, as if they were persons. Finally, those who do not accept the argument from potentiality will have to rely on the overflow principle to generate any restraints whatever on our behaviour towards the foetus, the infant, the curably or incurably mad, and even, it would seem, the deeply but reversibly unconscious (someone in dreamless sleep for example).
The application of the overflow principle to animals is as follows. Man is not only a rational being, but also an animal. More precisely, he is a rational animal, a being possessed of not only the attributes of thought and intention but also those of shape, size, health or disease, biological gender, and capacity for sensation. And while it is as rational beings that we are in the first place entitled to respect, the respect due to us as rational beings overflows to our animal nature, and to those creatures which, while 'dissociated from us by their want of reason' are nonetheless associated with us in sharing our animal capacities including the ability to suffer pain. If capacity for pain were the only feature of persons which entitled them to our consideration, then vegetarians would be right in attacking the person/animal distinction. But I see no reason to admit this premise.
This approach to animal suffering allows us to reach a happy compromise between the utilitarian and non-utilitarian approaches to the problem of cruelty to animals. Animal pain will be bad in itself, apart from any consequence of that pain to human beings, but the badness of that pain will derive from a moral principle whose ultimate reference is to persons. Thus the ethics proposed here is anthropocentric (or person-centred) though only mildly so.
(Philip E. Devine, "The Moral Basis of Vegetarianism," Philosophy 53 [October 1978]: 481-505, at 503-4 [italics in original; footnote omitted])