The issue as to who or what may be a possessor of rights is not simply a matter of academic, conceptual interest. Obviously, important conclusions follow from any answer. If, for instance, it is determined that gravely mentally defective human beings and monsters born of human parents are not the kinds of beings who may possess rights, this bears on how we may treat them. It does not settle such questions as to whether it is right to kill them if they are a burden or if they are enduring pointless suffering, but it does bear in an important way on such questions. Even if such beings cannot be possessors of rights it might still be wrong to kill them, but the case against killing those who endure pain is obviously easier to set out if they can be shown to be capable of possessing rights and in fact possess rights. Similarly, important conclusions follow from the question as to whether animals have rights. If they do, as Salt argued, it would seem an illegitimate invasion of animal rights to kill and eat them, if, as seems to be the case, we can sustain ourselves without killing animals. If animals have rights, the case for vegetarianism is prima facie very strong, and is comparable with the case against cannibalism.
(H. J. McCloskey, "Rights," The Philosophical Quarterly 15 [April 1965]: 115-27, at 122 [footnote omitted])