Sentience is the capacity to feel, perceive, or experience subjectively. 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"). In Eastern philosophy, sentience is a metaphysical quality of all things that require respect and care.
The prevailing scientific view today is that sentience is generated by specialized neural structures and processes - neuroanatomical, neurochemical, and neurophysiological. In more complex organisms these take the form of the central nervous system. According to the Cambridge Declaration on Consciousness (publicly proclaimed on 7 July 2012 at the Cambridge University), only those organisms within the animal kingdom that have these neural substrates are sentient.Sponges, placozoans, and mesozoans, with simple body plans and no nervous system, are the only members of the animal kingdom that possess no sentience.
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"--in other words, the ability to have states that it feels like something to be in. This is distinct from other features of the mind and consciousness, such as creativity, intelligence, sapience, self-awareness, and intentionality (the ability to have thoughts about something). Sentience is a minimalistic way of defining consciousness, which otherwise commonly and collectively describes sentience plus further features of the mind. These further features of consciousness may not be necessary for sentience, which rests on the capacity to feel sensations and emotions.
Some philosophers, notably Colin McGinn, believe that the physical process causing sentience to happen 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 cannot be explained. Other philosophers (such as Daniel Dennett) disagree, arguing that all aspects of consciousness will eventually be explained by science.
While it has been traditionally assumed that sentience and sapience are, in principle, independent of each other, there are criticisms of that assumption. One such criticism is about recognition paradoxes, one example of which is that an entity that cannot distinguish a spider from a non-spider cannot be arachnophobic. More generally, it is argued that since it is not possible to attach an emotional response to stimuli that cannot be recognized, emotions cannot exist independently of cognition that can recognize. The claim that precise recognition exists as specific attention to some details in a modular mind is criticized both with regard to data loss as a small system of disambiguating synapses in a module physically cannot make as precise distinctions as a bigger synaptic system encompassing the whole brain, and for energy loss as having one system for motivation that needs some built-in cognition to recognize anything anyway and another cognitive system for making strategies would cost more energy than integrating it all in one system that use the same synapses. Data losses inherent in all information transfer from more precise systems to less precise systems are also argued to make it impossible for any imprecise system to use a more precise system as an "emissary", as a less precise system would not be able to tell whether the outdata from the more precise system was in the interest of the less precise system or not.
The original studies by Ivan Pavlov that showed that conditioned reflexes in human children are more discriminating than those in dogs, human children salivating only at ticking frequencies very close to those at which food was served while dogs drool at a wider range of frequencies, have been followed up in recent years with comparative studies on more species. It is shown that both brain size and brain-wide connectivity contribute to make perception more discriminating, as predicted by the theory of a brain-wide perception system but not by the theory of separate systems for emotion and cognition.
Eastern religions including Hinduism, Buddhism, Sikhism, and Jainism recognise 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.
Sentience in Buddhism is the state of having senses. In Buddhism, there are six senses, 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. 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."
Granted, these animals do not have all the desires we humans have; granted, they do not comprehend everything we humans comprehend; nevertheless, we and they do have some of the same desires and do comprehend some of the same things. The desires for food and water, shelter and companionship, freedom of movement and avoidance of pain.
Animal-welfare advocates typically argue that any sentient being is entitled, at a minimum, to protection from unnecessary suffering, though animal-rights advocates may differ on what 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 [sic] 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--high intelligence; highly complex language; etc.--are not present in marginal cases such as mute humans, young children, deaf humans, and 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. His opponents accuse him of the same petitio principii.
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 recognising animals as sentient beings in biblically based faith traditions. The Interfaith Association of Animal Chaplains encourages animal ministry groups to adopt a policy of recognising 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 capable of experiencing pain and suffering.
The term "sentience" is not used by major artificial intelligence textbooks and researchers. It is sometimes used in popular accounts of AI to describe "human level or higher intelligence" (or artificial general intelligence).
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.