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.
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)[5] there exists some "marginal" human who is mentally handicapped in some way that would also meet the criteria for having no moral status.
[6]Daniel Dombrowski writes that the argument can be traced to Porphyry's third-century treatise On Abstinence from Eating Animals.
[5] In recent years, versions of the argument have been put forward by Peter Singer,[9] Tom Regan,[10] Evelyn Pluhar,[11] and Oscar Horta.
[12] 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 disabled people, Machan uses the analogy of a broken chair: ... classifications and ascriptions of capacities rely on the good sense of making certain generalizations.
[15] 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.
Arthur L. Caplan, in an article on the ethics of organ donation by infants with anencephaly – born essentially without a brain – before physical death, raises some points about the Argument from Marginal Cases.