The two main philosophical approaches to animal rights are utilitarian and rights-based. The former is exemplified by Peter Singer, and the latter by Tom Regan and Gary Francione. Their differences reflect a distinction philosophers draw between ethical theories that judge the rightness of an act by its consequences (consequentialism/teleological ethics, or utilitarianism), and those that focus on the principle behind the act, almost regardless of consequences (deontological ethics). Deontologists argue that there are acts we should never perform, even if failing to do so entails a worse outcome.[93] There are a number of positions that can be defended from a consequentalist or deontologist perspective, including the capabilities approach, represented by Martha Nussbaum, and the egalitarian approach, which has been examined by Ingmar Persson and Peter Vallentyne. The capabilities approach focuses on what individuals require to fulfill their capabilities: Nussbaum (2006) argues that animals need a right to life, some control over their environment, company, play, and physical health.[94] Stephen R. L. Clark, Mary Midgley, and Bernard Rollin also discuss animal rights in terms of animals being permitted to lead a life appropriate for their kind.[95] Egalitarianism favors an equal distribution of happiness among all individuals, which makes the interests of the worse off more important than those of the better off.[96] Another approach, virtue ethics, holds that in considering how to act we should consider the character of the actor, and what kind of moral agents we should be; Rosalind Hursthouse has suggested an approach to animal rights based on virtue ethics.[97] Mark Rowlands has proposed a contractarian approach Nussbaum (2004) writes that utilitarianism, starting with Jeremy Bentham and John Stuart Mill, has contributed more to the recognition of the moral status of animals than any other ethical theory.[100] The utilitarian philosopher most associated with animal rights is Peter Singer, professor of bioethics at Princeton University. Singer is not a rights theorist, but uses the language of rights to discuss how we ought to treat individuals. He is a preference utilitarian, meaning that he judges the rightness of an act by the extent to which it satisfies the preferences (interests) of those affected.[101] His position is that there is no reason not to give equal consideration to the interests of human and nonhumans, though his principle of equality does not require identical treatment. A mouse and a man both have an interest in not being kicked, and there are no moral or logical grounds for failing to accord those interests equal weight. Interests are predicated on the ability to suffer, nothing more, and once it is established that a being has interests, those interests must be given equal consideration.[102] Singer quotes the English philosopher Henry Sidgwick (18381900): "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."[72] Peter Singer: interests are predicated on the ability to suffer. Singer argues that equality of consideration is a prescription, not an assertion of fact: if the equality of the sexes were based only on the idea that men and women were equally intelligent, we would have to abandon the practice of equal consideration if this were later found to be false. But the moral idea of equality does not depend on matters of fact such as intelligence, physical strength, or moral capacity. Equality therefore cannot be grounded on the outcome of scientific investigations into the intelligence of nonhumans. All that matters is whether they can suffer.[103] Commentators on all sides of the debate now accept that animals suffer and feel pain, although it was not always so. Bernard Rollin, professor of philosophy, animal sciences, and biomedical sciences at Colorado State University, writes that Descartes' influence continued to be felt until the 1980s. Veterinarians trained in the US 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.[104] Scientific publications have made it clear since the 1980s 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.[105] The problem of animal suffering, and animal consciousness in general, arose primarily because it was argued that animals have no language. Singer writes that, if language were needed to communicate pain, it would often be impossible to know when humans are in pain, though we can observe pain behavior and make a calculated guess based on it. He argues that there is no reason to suppose that the pain behavior of nonhumans would have a different meaning from the pain behavior of humans