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Sentience is the ability to feel, perceive, or to experience subjectivity. Eighteenth-century philosophers used the concept to distinguish the ability to think (reason) from the ability to feel (sentience). In modern Western philosophy, sentience is the ability to experience sensations (known in philosophy of mind as "qualia"). For Eastern philosophy, sentience is a metaphysical quality of all things that requires respect and care. The concept is central to the philosophy of animal rights, because sentience is necessary for the ability to suffer, which is held to entail certain rights.
In the philosophy of consciousness, sentience can refer to the ability of any entity to have subjective perceptual experiences, or as some philosophers refer to them, "qualia". This is distinct from other aspects of the mind and consciousness, such as creativity, intelligence, sapience, self-awareness, and intentionality (the ability to have thoughts that mean something or are "about" something). Sentience is a minimalistic way of defining consciousness, which is otherwise commonly used to collectively describe sentience plus other characteristics of the mind. Voltaire compared the Hindu treatment of animals to how Europe's emperors & Popes treated even their fellow men, praising the former and heaping shame upon the latter; in the 17th century, Descartes, Pierre Gassendi, and Francis Bacon also advocated vegetarianism.
Some philosophers, notably Colin McGinn, believe that sentience will never be understood, a position known as "new mysterianism". They do not deny that most other aspects of consciousness are subject to scientific investigation but they argue that subjective experiences will never be explained; i.e., sentience is the only aspect of consciousness that can't be explained. Other philosophers (such as Daniel Dennett) disagree, arguing that such aspects of consciousness do not exist.
Eastern religions including Hinduism, Buddhism, Sikhism, and Jainism recognize non-humans as sentient beings. In Jainism and Hinduism, this is closely related to the concept of ahimsa, nonviolence toward other beings. In Jainism, all matter is endowed with sentience; there are five degrees of sentience, from one to five. Water, for example, is a sentient being of the first order, as it is considered to possess only one sense, that of touch. Man is considered a sentient being of the fifth order. According to Buddhism, sentient beings made of pure consciousness are possible. In Mahayana Buddhism, which includes Zen and Tibetan Buddhism, the concept is related to the Bodhisattva, an enlightened being devoted to the liberation of others. The first vow of a Bodhisattva states: "Sentient beings are numberless; I vow to free them."
Sentience in Buddhism is the state of having senses (sat + ta in Pali, or sat + tva in Sanskrit). In Buddhism, the senses are six in number, the sixth being the subjective experience of the mind. Sentience is simply awareness prior to the arising of Skandha. Thus, an animal qualifies as a sentient being.
In the philosophy of animal rights, sentience implies the ability to experience pleasure and pain. Animal-rights advocates typically argue that any sentient being is entitled, at a minimum, to the right not to be subjected to unnecessary suffering, though they may differ on what other rights (e.g., the right to life) may be entailed by simple sentience. Sentiocentrism describes the theory that sentient individuals are the center of moral concern.
The 18th-century philosopher Jeremy Bentham compiled enlightenment beliefs in Introduction to the Principles of Morals and Legislation, and he included his own reasoning in a comparison between slavery and sadism toward animals:
The French have already discovered that the blackness of the skin is no reason why a human being should be abandoned without redress to the caprice of a tormentor [see Louis XIV's Code Noir]... What else is it that should trace the insuperable line? Is it the faculty of reason, or, perhaps, the faculty of discourse? But a full-grown horse or dog is beyond comparison a more rational, as well as a more conversable animal, than an infant of a day, or a week, or even a month, old. But suppose the case were otherwise, what would it avail? The question is not Can they reason? nor, Can they talk? but, Can they suffer?
In the 20th century, Princeton University professor Peter Singer argued that Bentham's conclusion is often dismissed by an appeal to a distinction that condemns human suffering but allows non-human suffering, typically "appeals" that are logical fallacies (unless the distinction is factual, in which case the appeal is just one logical fallacy, petitio principii). Because many of the suggested distinguishing features of humanity—extreme intelligence; highly complex language; etc.—are not present in marginal cases such as young or mentally disabled humans, it appears that the only distinction is a prejudice based on species alone, which animal-rights supporters call speciesism—that is, differentiating humans from other animals purely on the grounds that they are human.
Gary Francione also bases his abolitionist theory of animal rights, which differs significantly from Singer's, on sentience. He asserts that, "All sentient beings, humans or nonhuman, have one right: the basic right not to be treated as the property of others."
Andrew Linzey, founder of the Oxford Centre for Animal Ethics in England, is known as a foremost international advocate for recognizing animals as sentient beings in biblically-based faith traditions. The Interfaith Association of Animal Chaplains encourages animal ministry groups to adopt a policy of recognizing and valuing sentient beings.
In 1997 the concept of animal sentience was written into the basic law of the European Union. The legally-binding protocol annexed to the Treaty of Amsterdam recognizes that animals are "sentient beings", and requires the EU and its member states to "pay full regard to the welfare requirements of animals".
The laws of several states include certain invertebrates such as cephalopods (octopuses, squids) and decapod crustaceans (lobsters, crabs) in the scope of animal protection laws, implying that these animals are also judged capable of experiencing pain and suffering.
David Pearce is a British philosopher of the negative utilitarian school of ethics. He is most famous for his advocation of the idea that there exists a strong ethical imperative for humans to work towards the abolition of suffering in all sentient beings.
Although the term "sentience" is avoided by major artificial intelligence textbooks and researchers, it is sometimes used in popular accounts of AI to describe "human level or higher intelligence" (or strong AI). This is closely related to the use of the term in science fiction.
In science fiction, an alien, android, robot, hologram, or computer described as sentient is usually treated as a fully human character, with similar rights, qualities, and capabilities as any other character. Foremost among these properties is human level intelligence (i.e. "sapience"), but sentient characters also typically display desire, will, consciousness, ethics, personality, insight, and many other human qualities. Sentience is being used in this context to describe an essential human property that brings all these other qualities with it. This often leads to the word being incorrectly used - for example, an episode of Star Trek: The Next Generation has a character describing his cat as "not sentient", while by the real world definition a cat is sentient. The words "sapience", "self-awareness", and "consciousness" are used in similar ways in science fiction.
The sentience quotient concept was introduced by Robert A. Freitas Jr. in the late 1970s. It defines sentience as the relationship between the information processing rate of each individual processing unit (neuron), the weight/size of a single unit, and the total number of processing units (expressed as mass). It was proposed as a measure for the sentience of all living beings and computers from a single neuron up to a hypothetical being at the theoretical computational limit of the entire universe. On a logarithmic scale it runs from −70 up to +50.