Sentience is the capacity to feel, or have conscious experience. (Sometimes the term is used more narrowly to refer to the capacity to feel pleasure or pain.) It is generally accepted that possessing this capacity is a necessary condition for counting as a moral patient.
Philosophers and scientists discuss three broad hypotheses on what entities are sentient.
First, it is possible that only humans are sentient. 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 nonhuman animals are sentient (see animal sentience). 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 beings other than human and nonhuman animals are sentient (see artificial sentience). 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....