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, senile people, the comatose, and cognitively disabled people have direct moral status, non-human animals must have a similar status, since there is no known morally relevant characteristic 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.

Although various cases are made for it, Raymond Frey has described the argument from marginal cases collectively as 'one of the most common arguments in support of an equal value' of animals' lives.

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 her. 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 (either capacities, e.g. language, consciousness, the ability to have moral responsibilities towards others; or relations, e.g. sympathy or power relations) 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's third-century treatise On Abstinence from Eating Animals. Danish philosopher Laurids Smith who was familiar with the arguments of Wilhelm Dietler argued against the idea that animals cannot possess rights because they cannot understand the ideas of right and duty. He reasoned that if this was correct then children and mentally ill people would not have rights either. Other 18th-century philosophers who presented similar arguments include David Hume and Jeremy Bentham.

In recent years, versions of the argument have been put forward by Peter Singer, Tom Regan, Evelyn Pluhar, and Oscar Horta.