Sentience

Sentience is the capacity of a living thing to experience sensations such as pleasure or pain. As such, it should not be confused with sapience, which is the capacity for advanced thinking, comprehension, rationality and abstraction.

In principle, almost any life form with a nervous system can be considered sentient, including almost all animal life on Earth, possibly with some exceptions.

Classification
In fiction as well as scientific speculation, it is possible to classify living things according to their sapience and sentience capabilities:
 * 1) Non-sentient and non-sapient: should include most types of unintelligent vegetation, some microbes and extremely-simple animals such as sponges or placozoans. Also includes some kinds of undead beings such as "zombies" as well as machines and artificial creatures which are developed enough to be considered alive, but not self-aware.
 * 2) Sentient but non-sapient: should include most higher animals and the equivalent. These are creatures that are capable of experiencing sensations, but not capable of advanced thinking and metacognition.
 * 3) Sentient and sapient: should include pretty much all intelligent races, such as humans and other life forms which display the capacity to think as well as to experience sensations.
 * 4) Sapient but non-sentient: probably the rarest category, should be reserved for life forms which are capable of advanced thinking, but not of experiencing sensations. Many types of synthetic and robotic creatures may fall here, notably including the original bearers of the name "Robot", from Karel Capek's 1920 play R.U.R..