Peter Singer defends animal rights from their ability feel pain. Since animals have no language, leading scientists argue that it is impossible to know when an animal is suffering. This situation may change as increasing numbers of chimps are taught sign language, although skeptics question whether their use of it portrays real understanding. Singer writes that, following the argument that language is needed to communicate pain, it would often be impossible to know when humans are in pain. All we can do is observe pain behavior, he writes, and make a calculated guess based on it. As Ludwig Wittgenstein argued, if someone is screaming, clutching a part of their body, moaning quietly, or apparently unable to function, especially when followed by an event that we believe would cause pain in ourselves, that is in large measure what it means to be in pain. Singer argues that there is no reason to suppose animal pain behavior would have a different meaning.
Tom Regan argues that animals are what he calls "subjects-of-a-life," and as such are bearers of rights. He argues that, because the moral rights of humans are based on their possession of certain cognitive abilities, and because these abilities are also possessed by at least some non-human animals, such animals must have the same moral rights as humans. Although only humans act as moral agents, both marginal-case humans, such as infants, and at least some non-humans must have the status of "moral patients." Moral patients are unable to formulate moral principles, and as such are unable to do right or wrong, even though what they do may be beneficial or harmful. Only moral agents are able to engage in moral action. Animals for Regan have "inherent value" as subjects-of-a-life, and cannot be regarded as a means to an end.
Some critics of Regan, like Roger Scruton, argue that rights also imply obligations, which animals cannot be forced to have (although Scruton disagrees with Regan over the issue of rights, he opposes factory farming.
Abolitionism: It falls within the framework of the rights-based approach, though it regards only one right as necessary: the right not to be owned. Abolitionists argue that the key to reducing animal suffering is to recognize that legal ownership of sentient beings is unjust and must be abolished. The most prominent of the abolitionists is Gary Francione, professor of law and philosophy at Rutgers School of Law-Newark. He argues that focusing on animal welfare may actually worsen the position of animals, because it entrenches the view of them as property, and makes the public more comfortable about using them.
