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.
Overview of the argument
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.
A counter-argument is the Argument from Species Normality (a term coined by David Graham), proposed by Tibor Machan. In considering the rights of children or the disabled, Machan uses the analogy of a broken chair:
... classifications and ascriptions of capacities rely on the good sense of making certain generalizations. One way to show this is to recall that broken chairs, while they aren't any good to sit on, are still chairs, not monkeys or palm trees. Classifications are not something rigid but something reasonable. While there are some people who either for a little or longer while – say when they're asleep or in a coma – lack moral agency, in general people possess that capacity, whereas non-people don't. So it makes sense to understand them having rights so their capacity is respected and may be protected. This just doesn't work for other animals.
David Graham interprets this to mean that if most of a species' members are moral agents then any member has the same rights and protections as the species. In brief, "The moral status of an individual depends on what is normal for that individual's species."
James Rachels has responded to Machan that if one adopts the idea that individuals of a species must be treated according to what is normal for that species, then it would imply a chimp that somehow acquired the ability the read and write should not enter a university since it is not "normal" behavior for a chimpanzee.
A related counterargument from Roderick Long is that a being can obtain moral agency by developing a rational capacity, and from there on has full moral agency even if this capacity is lost or diminished:
That is why a cow has no rights, though a human being reduced to the mental level of a cow does have them. There's something wrong with the human; there's nothing wrong with the cow. One might say that in the case of the cow-minded human, there's a blank spot where her moral agency is supposed to be, and someone else can step into that blank spot and act as an agent on her behalf. But in the cow there's no blank spot.
- "The argument from species overlap". Animal Ethics. Retrieved 2015-02-05.
- Dombrowski, Daniel (1997). Babies and Beasts: The Argument from Marginal Cases. University of Illinois Press.
- Harter, S. (1983). Developmental perspectives on the self-system. In P. H. Mussen (Ed.), Handbook of child psychology, (4th ed., Vol. 4, (pp. 275–385). New York: Wiley.
- Singer, Peter. Animal Liberation. New York: HarperCollins, 2002. Print.
- Dombrowski, Daniel A. "Vegetarianism and the Argument from Marginal Cases in Porphyry". Journal of the History of Ideas 45.1 (1984): 141–143.
- Rachels, James. Created from Animals: The Moral Implications of Darwinism. Oxford University Press, 1991.
- Machan, Tibor. Putting Humans First: Why We Are Nature's Favorite.
- A Libertarian Replies to Tibor Machan's 'Why Animal Rights Don't Exist'
- Animal Rights and Human Obligations, p. 100.
- Why Fur Is Not Murder
- Dombrowski, Daniel. Babies and Beasts: The argument from marginal cases University of Illinois Press, 1997, ISBN 978-0-252-06638-2.
- Animal Liberation: A New Ethics for our Treatment of Animals, Peter Singer, New York Review/Random House, New York, 1975; Cape, London, 1976; Avon, New York, 1977; Paladin, London, 1977; Thorsons, London, 1983. Harper Perennial Modern Classics, New York, 2009.
- Articles by James Lindemann Nelson and Evelyn Pluhar in Bekoff, Marc (1998-06-30). Encyclopedia of Animal Rights and Animal Welfare (1 ed.). Greenwood Press. pp. 237–241. ISBN 0-313-29977-3.