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Why do arguments based on language or free will fail to separate all humans from all animals?

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Three PETA protesters in animal masks and green clothing hold signs reading No Fur at a demonstration against fur in clothing at a shopping district in Seoul, South Korea. — Animal rights A PETA (People for the Ethical Treatment of Animals) demonstration in Seoul, South Korea, against the use of animals for fur in clothing.

Arguments based on language or free will (often categorized as moral agency or autonomy) fail to separate all humans from all animals due to the problem of "marginal cases." Defenders of speciesism often argue that these specific capabilities justify prioritizing human interests over those of animals. However, this position is challenged by the fact that not all humans possess these traits to the same degree.1

For instance, infants and individuals with certain intellectual disabilities or cognitive impairments may lack the level of language use, rationality, or autonomous moral agency that is cited as the basis for human moral superiority. If these capabilities are the necessary criteria for moral consideration, then these "marginal" humans would logically fall into the same category as animals. Conversely, if one maintains that all humans deserve equal moral consideration regardless of these capacities, then it becomes difficult to justify excluding animals who share similar interests, such as the capacity to feel pain.1

Ultimately, using these traits to draw a sharp moral line between species leads to a dilemma: either the interests of some humans are no more important than those of some animals, or the interests of some animals are as significant as those of humans.1