There are two main philosophical approaches to the issue of animal rights: a utilitarian and a rights-based one. The former is exemplified by Peter Singer, professor of bioethics at Princeton, and the latter by Tom Regan, professor emeritus of philosophy at North Carolina State University, and Gary Francione, professor of law and philosophy at Rutgers School of Law-Newark
Their differences reflect a distinction philosophers draw between ethical theories that judge the rightness of an act by its consequences (called consequentialism, teleological ethics, or utilitarianism, which is Singer's position), and those that judge acts to be right or wrong in themselves, almost regardless of consequences (called deontological ethics, of which Regan and Francione are adherents). A consequentialist might argue, for example, that lying is wrong if the lie will make someone unhappy. A deontologist would argue that lying is wrong in principle.
Within the animal rights debate, Singer does not believe there are such things as natural rights and that animals have them, although he uses the language of rights as shorthand for how we ought to treat individuals. Instead, he argues that, when we weigh the consequences of an act in order to judge whether it is right or wrong, the interests of animals, primarily their interest in avoiding suffering, ought to be given equal consideration to the similar interests of humans. That is, where the suffering of one individual, human or non-human, is equivalent to that of any other, there is no moral reason to award more weight to either one of them. Regan's and Francione's approaches, on the other hand, are not driven by the weighing of consequences. Regan believes that animals are what he calls "subjects-of-a-life," who have moral rights for that reason, and that moral rights ought not to be ignored. Francione argues that animals have one moral right, and need one legal one: the right not to be regarded as property. All else will follow from that one paradigm shift, he argues.
Singer is an act utilitarian, or more specifically a preference utilitarian, meaning that he judges the rightness of an act by its consequences, and specifically by the extent to which it satisfies the preferences of those affected, maximizing pleasure and minimizing pain. (There are other forms of utilitarianism, such as rule utilitarianism, which judges the rightness of an act according to the usual consequences of whichever moral rule the act is an instance of.)
Singer's position is that there are no moral grounds for failing to give equal consideration to the interests of human and non-humans. His principle of equality does not require equal or identical treatment, but equal consideration of interests. A mouse and a man both have an interest in not being kicked down the street, because both would suffer if so kicked, and there are no moral or logical grounds, Singer argues, for failing to accord their interests in not being kicked equal weight. Singer quotes the English philosopher Henry Sidgwick: "The good of any one individual is of no more importance, from the point of view ... of the Universe, than the good of any other." This reflects Jeremy Bentham's position: "[E]ach to count for one, and none for more than one." Unlike the position of a man or a mouse, a stone would not suffer if kicked down the street, and therefore has no interest in avoiding it. Interests, Singer argues, are predicated on the ability to suffer, and nothing more, and once it is established that a being has interests, those interests must be given equal consideration. The issue of the extent to which animals can suffer is therefore key.
 Animal suffering
Singer writes that commentators on all sides of the debate now accept that animals suffer and feel pain, although it was not always so. Bernard Rollin, a philosopher and professor of animal sciences, writes that Descartes' influence continued to be felt until the 1980s. Veterinarians trained in the U.S. before 1989 were taught to ignore pain, he writes, and at least one major veterinary hospital in the 1960s did not stock narcotic analgesics for animal pain control. In his interactions with scientists, he was often asked to "prove" that animals are conscious, and to provide "scientifically acceptable" evidence that they could feel pain.
Singer writes that scientific publications have made it clear over the last two decades that the majority of researchers do believe animals suffer and feel pain, though it continues to be argued that their suffering may be reduced by an inability to experience the same dread of anticipation as humans, or to remember the suffering as vividly. In the most recent edition of Animal Liberation, Singer cites research indicating that animal impulses, emotions, and feelings are located in the diencephalon, pointing out that this region is well developed in mammals and birds. Singer also relies on the work of Richard Sarjeant to support his position. Sarjeant pointed out that non-human animals possess anatomical complexity of the cerebral cortex and neuroanatomy that is nearly identical to that of the human nervous system, arguing that, "[e]very particle of factual evidence supports the contention that the higher mammalian vertebrates experience pain sensations at least as acute as our own. To say that they feel less because they are lower animals is an absurdity; it can easily be shown that many of their senses are far more acute than ours."
The problem of animal suffering, and animal consciousness in general, arises primarily because animals have no language, leading scientists to 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.
 Equality a prescription, not a fact
|“||They talk about this thing in the head; what do they call it? ["Intellect," whispered someone nearby.] That's it. What's that got to do with women's rights or Negroes' rights? If my cup won't hold but a pint and yours holds a quart, wouldn't you be mean not to let me have my little half-measure full? — Sojourner Truth||”|
Singer argues that equality between humans is not based on anything factual, but is simply a prescription. Humans do, in fact, differ in many ways. If the equality of the sexes were based on the idea, for example, that men and women are in principle capable of being equally intelligent, but this was later found to be false, it would mean we would have to abandon the practice of equal consideration. But equality of consideration is based on a prescription, not a description. It is, Singer writes, a moral idea, not an assertion of fact. He quotes President Thomas Jefferson, the principal author in 1776 of the American Declaration of Independence: "Because Sir Isaac Newton was superior to others in understanding, he was not therefore lord of the property or persons of others."
 Rights-based approach
 Tom Regan: Subjects-of-a-life
Tom Regan argues in The Case for Animal Rights and Empty Cages that non-human 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. This is also called the "direct duty" view. His theory does not extend to all sentient animals but only to those that can be regarded as subjects-of-a-life. He argues that all normal mammals of at least one year of age would qualify in this regard. Whereas Singer is primarily concerned with improving the treatment of animals and accepts that, in some hypothetical scenarios, individual animals might be used legitimately to further human or non-human ends, Regan believes we ought to treat non-human animals as we would humans. He applies the strict Kantian ideal (which Kant himself applied only to humans) that they ought never to be sacrificed as a means to an end, and must be treated as ends in themselves.
 Gary Francione: Abolitionism
Abolitionism 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.
Francione calls animal rights group who pursue animal welfare issues, such as People for the Ethical Treatment of Animals, the "new welfarists," arguing that they have more in common with 19th-century animal protectionists than with the animal rights movement. He argues that there is no animal rights movement in the United States.
 Carl Cohen
Critics such as Carl Cohen, professor of philosophy at the University of Michigan and the University of Michigan Medical School, oppose the granting of personhood to animals, arguing that rights holders must be able to distinguish between their own interests and what is right. "The holders of rights must have the capacity to comprehend rules of duty governing all, including themselves. In applying such rules, [they] ... must recognize possible conflicts between what is in their own interest and what is just. Only in a community of beings capable of self-restricting moral judgments can the concept of a right be correctly invoked."
Cohen rejects Singer's argument that, since a brain-damaged human could not make moral judgments, moral judgments cannot be used as the distinguishing characteristic for determining who is awarded rights. Cohen writes that the test for moral judgment "is not a test to be administered to humans one by one," but should be applied to the capacity of members of the species in general.
 Posner–Singer debate
Judge Richard Posner of the United States Court of Appeals for the Seventh Circuit debated the issue of animal rights with Peter Singer on Slate. Posner argues that his moral intuition tells him "that human beings prefer their own. If a dog threatens a human infant, even if it requires causing more pain to the dog to stop it, than the dog would have caused to the infant, then we favour the child. It would be monstrous to spare the dog."
Singer challenges Posner's moral intuition by arguing that formerly unequal rights for gays, women, and certain races were justified using the same set of intuitions. Posner replies that equality in civil rights did not occur because of ethical arguments, but because facts mounted that there were no morally significant differences between humans based on race, sex, or sexual orientation that would support inequality. If and when similar facts emerge about the difference, or lack thereof, between humans and animals, the differences in rights will erode too. But facts will drive equality, not ethical arguments that run contrary to instinct, he argues.
Posner calls his approach "soft utilitarianism," in contrast to Singer's "hard utilitarianism." He argues: "The "soft" utilitarian position on animal rights is a moral intuition of many, probably most, Americans. We realize that animals feel pain, and we think that to inflict pain without a reason is bad. Nothing of practical value is added by dressing up this intuition in the language of philosophy; much is lost when the intuition is made a stage in a logical argument. When kindness toward animals is levered into a duty of weighting the pains of animals and of people equally, bizarre vistas of social engineering are opened up."
 Roger Scruton
|“||Considerate la vostra semenza: ||”|
The British philosopher Roger Scruton argues that rights imply obligations. Every legal privilege, he writes, imposes a burden on the one who does not possess that privilege: that is, "your right may be my duty." Scruton therefore regards the emergence of the animal rights movement as "the strangest cultural shift within the liberal worldview," because the idea of rights and responsibilities are, he argues, distinctive to the human condition, and it makes no sense to spread them beyond our own species.
He accuses animal rights advocates of "pre-scientific" anthropomorphism, attributing traits to animals that are, he says, Beatrix Potter-like, where "only man is vile." It is within this fiction that the appeal of animal rights lies, he argues. The world of animals is non-judgmental, filled with dogs who return our affection almost no matter what we do to them, and cats who pretend to be affectionate when, in fact, they care only about themselves. It is, he argues, a fantasy, a world of escape.