Sentience is the ability to sense. It is separate from, and not dependent on, aspects of consciousness.
Philosophy and sentience
Many philosophers, notably Colin McGinn, believe that sentience cannot ever 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, to 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 no really 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:
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... 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?"
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 an irrational prejudice on the basis of 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.
On the other hand, some have argued that modern science cannot determine exactly where sentience begins, going from bacteria to animals. This would pose considerable complications for a theory of unnecessary suffering. Others take no objection with the conclusion that it's wrong to cause unnecessary suffering, but contend that on this issue the moral concept of right/wrong shouldn't mirror human nature but should instead be modelled from nature. Since animals routinely kill each other and inflict (at times unnecessary) suffering on each other, then as part of animalia it wouldn't be wrong for us to also. This is a view most of the world's population follows, whether intentionally acknowledging it or not. Therefore, the reason the rules of nature regarding killing aren't applicable towards other humans is because we are then dealing with the human realm. Our own psychology and the collective sociology make it unfavorable (ie. less safety, added stress, reduced efficiency) to partake in killing other humans. Seen in this light, it would not be speciesism to kill animals but spare humans, but instead an outgrowth of humans' (as a species) naturalistic adaptation while observing all natural ethics regarding suffering.
Eastern religions including Hinduism, Buddhism, 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 the 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."
Book about A Theory of Sentience Readership: Philosophers, psychologists, and neuroscientists interested in sensation and perception. Authors, Austen Clark, Professor of Philosophy, University of Connecticut, Storrs