Argument from marginal cases
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The argument from marginal cases is a philosophical argument within animal rights theory. The argument compares people with impaired or otherwise below-normal abilities to animals and concludes that they should be treated similarly, namely that the same protections and moral considerations should be extended to animals as are applied to these "marginal case" humans.
The argument from marginal cases begins with the premise that some human beings are "marginal" in that they lack certain abilities or display these abilities to a lesser degree than others. Some human beings, for example, are mentally retarded. However, we regularly and emphatically extend protection to these individuals, as we do to all other humans within our society.
The other key premise of the argument is that animals are relevantly similar (say, in their capacity to suffer) to such individuals. It follows that we ought to extend the same protection to animals as we extend to "marginal" human beings (at least insofar as suffering is concerned). Otherwise, we would be justified in subjecting the disabled to the same cruelties we visit upon animals.
- P1: If no morally relevant difference between marginal human beings and some non-human animals exists, and some marginal human beings have moral status, then some non-human animals have moral status.
- P2: No morally relevant difference exists between marginal human beings and some non-human animals.
- C1: Therefore, some non-human animals have moral status.
The argument is also popularly rendered in the form of a reductio ad absurdum argument, in which various properties — capacity for rational thought, grasp of morality, self-awareness etc. — on which the moral status of humans is purported to be based (and which are lacking in animals) are 'disproven' by bringing up "marginal" humans who lack said properties and arguing that it'd be absurd to treat these humans with the same moral disregard as we do animals. It is then noted that any moral-status-awarding characteristic which is shared by all human beings will inevitably also be shared with some non-human animals.
The argument's most basic flaw is defining disabled people, coma victims and — in some iterations — young children as "marginal cases", an opaque label that is rarely (if ever) applied to them otherwise. The implication that these are not proper human beings or members of society is rather disturbing and the same line of reasoning could be used to argue against respecting and protecting these people's rights.
Connected with this is the assumption that human rights are defined by ability, such as the ability to suffer, whereas human rights proponents define the rights instead as belonging to all humans and not to anything other than humans. The Universal Declaration of Human Rights, for example, begins with the statement that "recognition of the inherent dignity and of the equal and inalienable rights of all members of the human family is the foundation of freedom, justice and peace in the world".
There are legitimate questions about why human rights are so defined, and a variety of explanations ranging from the religious (God endowed humankind with its special status) to the genetic (we have stronger obligations to members of our own species than any other) to the social contract (we have a commitment to protect members of our society). Animals do not meet any of these criteria (though some animal rights proponents argue, rather dubiously, that pets and farm animals are members of our society).
Broadly speaking, all of these ideas designed to rationalize human rights being justified but animal rights not do not protect intelligent life outside humanity, and even more disturbingly, there is no logical reason why rights should be extended specifically at the species level but withheld outside it. For example, Homo Neanderthalensis was never part of modern society, but would not be badly out of place in it. If we include Neanderthals, then by genetics, it is only right that we include Hiedelbergensis. If we go further and further, we reach our common ancestor with Chimpanzees, and so should probably include the entire clade.
If instead we declare that, despite close genetic and capability relations to Sapiens, Neanderthals are fine to slaughter, enslave, or do anything else with, we ought also exclude Hiedelbergensis, and are forced to set a boundary. There is no reason other than convenience to include all living members of our species but not deceased clades, so this opens the door to theories of human rights which discriminate against genetically "inhuman" members of H. Sapiens.
If instead we argue that rights come from the social contract, then this justifies almost anything done to foreigners so long as it does not bring them into society. For example, having a genocidal war would certainly be okay, since the people are even less under the dominion of the government than wild animals within one's own lands. Using the social contract as the basis for human rights effectively means that anyone less a part of your society in particular than a farm animal on a farm in said society is morally irrelevant. It also implies that unprovoked murder would not be wrong in an anarchic society.
In general, attempting to rationalize away the question of animal rights is little more than that: rationalization so that one can kill animals with impunity by ensuring that any entity one does not personally value is exempt from all moral calculus "by definition."
 Human rationality
Certainly the concept of human rights has a lot to do with humanity's capacity (as a species) for rationality, abstract thought, and empathy — even if we extend the same rights to members of our species who are lacking in these capacities.
If we were confronted with a species or entity with capacities for rationality similar to our own — for example, a newly discovered extraterrestrial species or, arguably, a highly advanced artificial intelligence — we would be faced with the dilemma of whether to accord them the same rights as humans. This is a common theme within science fiction.
None of the current species on Earth have the cognitive abilities to place us in this dilemma, but the argument from marginal cases subverts this by lowering the bar: extending human rights not only to species with the cognitive abilities of a typical human but to species with cognitive abilities comparable to those of a human with severely reduced cognitive abilities.
Hence, instead of applying the reasonable principle of "if it behaves like a human, treat it like a human", which we could use in the hypothetical case of dealing with a new species, extraterrestrial or AI, the argument from marginal cases applies the principle of "if it behaves like a human with impaired capacities, treat it like a human with impaired capacities". This could be applied not only to animals but to even the simplest of AI computer programs, such as a chatterbot, chess computer or a non-player character in any video game.