Sentience

Overview
Sentience is the ability to feel or perceive subjectively.

Philosophy and sentience
Many philosophers, notably Colin McGinn, believe that sentience will never be understood, no matter how much progress is made by neuroscience in understanding the brain. Holders of this position are called New Mysterians. They do not deny that most other aspects of consciousness are subject to scientific investigation, from creativity to sapience and self-awareness. New Mysterians believe that only sentience cannot be comprehensively understood by science. There continues to be much debate among philosophers, with many adamant that there is really no hard problem with sentience whatsoever.

Non-human animal rights and sentience
In the philosophy of animal rights, sentience is commonly seen as the ability to experience suffering. The 18th century philosopher Jeremy Bentham raised the issue of non-human suffering and sadism in An Introduction to the Principles of Morals and Legislation:

As Peter Singer argues, this is often dismissed by appeal to a distinction that condemns humans suffering but allows non-human suffering. However, as 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."

Science fiction
In science fiction, an alien, android, robot or computer who is described as "sentient" is often ascribed qualities such as will, desire, consciousness, ethics, personality, intelligence, insight, and so on. Sentience is being used in this context to describe an essential human property that brings all these other qualities with it.

Some science fiction plot lines explore ethical concerns, analogous to the concerns of advocates of animal rights. In an episode of Star Trek, "The Measure of a Man," Data, a sentient android, takes legal action to prove that he has the same rights as a human being. 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 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.

Eastern religion
Eastern religions including Hinduism, Buddhism, Sikhism, Islam, 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 six degrees of sentience, from one to six. 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 sixth 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). And the senses are six in number, the sixth being the mind aka 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 a sentient), 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 lanaguage), oculesics (eye language) and proxemics (distance).

Western religion
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.

Sentience quotient
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 (bit/s) 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.

Sentience vs. Sapience
The word sentient is often confused with the word sapient, which can connote knowledge, consciousness, or apperception. The root of the confusion is that the word conscious has a number of different usages in the English language. The two words can be distinguished by looking at their Latin roots: sentire, "to feel"; and sapere, "to know".