Sentience is the capacity to feel, or have subjective experiences. According to many views in normative ethics, the possession of this capacity is a necessary condition for counting as a moral patient. (Sometimes the term is used more narrowly to refer to the capacity to feel pleasure or pain.)
All plausible theories of value hold that pleasure and the avoidance of pain have value. It is therefore highly important to ask which entities are capable of experiencing pleasure and pain.
Philosophers and scientists discuss three broad hypotheses on this issue.
First, it is possible that only humans feel pleasure and pain. This is currently an uncommon view, although it has a long history. For instance, the 17th century philosopher René Descartes put forward influential arguments to the effect that animals lack internal experience, and until several decades ago animal experimenters and veterinarians were taught to disregard apparent pain responses.
Second, it is possible that only sufficiently advanced non-human animals feel pleasure and pain. This is the most common view: that other creatures such as chimpanzees, dogs, and pigs also have internal experiences, but that there is some cut-off point beyond which species such as clams, jellyfish, and sea-sponges lie. A conservative cut-off of this sort might include only primates, and a liberal cut-off might go so far as to include insects.
Third, it is possible that animals are not the only beings capable of experiencing pleasure and pain. Some philosophers argue that sufficiently advanced artificial intelligence would be capable of experiencing these feelings, or that sufficiently detailed computer simulations of people would have the same experiences that flesh-and-blood people do. Also, it is not necessarily inconceivable that plants, relatively simple machines, or even fundamental physical processes, can experience pleasure or pain, although there are very few proponents of these views.
As a final point, it is not necessarily the case that all beings’ experiences are of equal importance, or even that a being which can experience pain is necessarily a moral patient. For instance, one common ethical view is that animal pain matters less than human pain even when it is of the same intensity.
Allen, Colin (2004) Animal pain, Noûs, vol. 38, pp. 617–643.
A summary some of the academic discussion around the existence of pain in animals.
Tomasik, Brian (2009) Do bugs feel pain?, Essays on Reducing Suffering, April 7.