Sentience is the ability to feel or perceive subjectively. The term is used in philosophy (particularly in the philosophy of animal ethics and in eastern philosophy) as well as in science fiction and (occasionally) in the study of artificial intelligence. In each of these fields the term is used slightly differently.
In eastern philosophy, sentience is a metaphysical quality of all things that requires our respect and care. In science fiction, sentience is "personhood": the essential quality that separates humankind from machines or lower animals. Sentience is used in the study of consciousness to describe the ability to have sensations or experiences, known to some Western academic philosophers as "qualia".
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.
In the 17th century Thomas Tryon, a self-proclaimed Pythagorean, raised the issue of non-human suffering. Soon thereafter, many philosophers used the anatomical discoveries of the Enlightenment as a reason to include animals in what philosophers call "sympatheia," the principle of who or what deserves sympathy. Benjamin Franklin's autobiography identifies Tryon's writings as an influence in his decision to try vegetarianism; later in the book, he reverts to eating meat while still following Tryon's basic philosophy. Joseph Ritson coupled Tryon's work with Rousseau's for "Essay on Abstinence from Animal Food" as many Rousseauists became vegetarian. Voltaire compared the Hindu treatment of animals to how Europe's emperors and 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.
The 18th century philosopher Jeremy Bentham compiled Enlightenment beliefs in An 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. 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 non-human 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 recognises 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 to be capable of experiencing pain and suffering.
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. Some sources reserve the term "sapience" for human level intelligence and make a distinction between "sentience" and "sapience". The Author Iain Banks uses "Sentient machines" in his The Culture Novels.
In science fiction, an alien, android, robot, hologram, or computer who is 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 (see above), 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. The words "sapience", "self-awareness", and "consciousness" are used in similar ways in science fiction.
Some science fiction plot lines explore ethical concerns analogous to the concerns of advocates of animal rights. In an episode of Star Trek: The Next Generation, "The Measure of a Man," Data, a sentient android, takes legal action to prove that he has the same rights as a human being. In the Star Trek: Voyager episode Author, Author the Doctor, a holographic program by nature, fights for his rights as a sentient lifeform. The film Artificial Intelligence: A.I. considers a machine in the form of a small boy which has been given the ability to feel human emotions, including the capacity to suffer. In these examples, sentience plays the same role as it does in the philosophy of animal rights.
The word sentient is also used to describe the Agent programs in The Matrix, however in this case it only means that these programs are self-aware, self-governing, and of human-level intelligence—no individuality or personality is implied. However, the character behaves in a way that expresses personality, antipathy and emotion.
In many science fiction works sentience is often used as a synomym for sapience meaning "human-level or higher intelligence". But others make a distinction; for example, in David Brin's Uplift stories, the Tandu are undoubtedly sapient (both technologically skilled and cunning) but only marginally sentient, since they regard other races and sometimes other Tandu mainly as potential prey.
In the science fiction MMORPG "Jumpgate" "sentients" are special specimens of the otherwise AI-species "Conflux" which are controlled by employers of the game company in special events to create a greater challenge for the players than the usual computer-controlled NPCs.
Eastern religions including Hinduism, Buddhism, Sikhism, and Jainism recognize nonhumans 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 first order, as it is considered to possess only one sense, that of touch. Man is considered to be 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 is, from a Buddhist perspective, 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 mind or consciousness, just as consciousness is in the whole body. Sentience, then, is the ability to sense / experience pain and pleasure, make conscious choices, such as abstaining from action, speech, speculation, etc. Thus, while an animal qualifies as a sentient being, a computer doesn't, for at least two reasons: (a) Even if it makes intelligent decisions (which no computer will ever be capable of without sentience), it has to be programmed by an outside agent (human or even a super-computer), whereas a sentient being is self-directed, and (b) a computer must always perform using instructions in order to communicate, whereas a sentient being, can still express in silence - through kinesics (body language), oculesics (eye language) and proxemics (distance).
In the philosophy of consciousness, "sentience" can refer to the ability of any entity to have subjective perceptual experiences, or "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.
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 all aspects of consciousness will eventually yield to scientific investigation.
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 beings living and computer 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.
The word sentient is often confused with the word sapient, which is essentially a confusion between simple perception or sensation and intelligence or thinking.