Argument from marginal cases

The argument from marginal cases (also known as the argument from species overlap) is a philosophical argument within animal rights theory regarding the moral status of non-human animals. Its proponents hold that if human infants, the senile, the comatose, and the cognitively disabled have direct moral status, animals must have a similar status, since there is no known morally relevant ability that those marginal-case humans have that animals lack. "Moral status" may refer to a right not to be killed or made to suffer, or to a general moral requirement to be treated in a certain way.

The argument from marginal cases takes the form of a proof by contradiction. It attempts to show that you cannot coherently believe both that all humans have moral status, and that all non-humans lack moral status.

Consider a cow. We ask why it is acceptable to kill this cow for food – we might claim, for example, that the cow has no concept of self and therefore it cannot be wrong to kill it. However, many young children may also lack this same concept of "self". So if we accept the self-concept criterion, then we must also accept that killing children is acceptable in addition to killing cows, which is considered a reductio ad absurdum. So the concept of self cannot be our criterion.

The proponent will usually continue by saying that for any criterion or set of criteria (e.g. language, consciousness, ethics) there exists some "marginal" human who is mentally handicapped in some way that would also meet the criteria for having no moral status. Peter Singer phrases it this way:

The catch is that any such characteristic that is possessed by all human beings will not be possessed only by human beings. For example, all human beings, but not only human beings, are capable of feeling pain; and while only human beings are capable of solving complex mathematical problems, not all humans can do this.

Daniel Dombrowski writes that the argument can be traced to Porphyry in the third century AD.James Rachels has argued that the theory of evolution implies that there is only a gradient between humans and other animals, and therefore marginal-case humans should be considered similar to non-human animals.

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