Who Are The Sentient Beings?
Buddhist teachings, many times the term 'sentient beings' is mentioned without
a definition being offered for its meaning. In the boddhisattva vow, the Buddhist
practitioner takes an oath to liberate all sentient beings. But who are these
sentient beings? Are they the same as living beings?
Before attempting a definition, it is necessary to start with an interesting story. Alan Turing,one of the pioneers of the computer industry, developed a test to determine when a computer would reach the level of intelligence of a human being. The test started with a person in a closed room with two screens and keyboards in front of him. The subject typed questions in both keyboards and received answers on each of the screens. On the other side of the wall, there was a real person answering the questions and dialoging with the subject on one screen, while a programmed computer sent responses to the second screen. The moment the subject was unable to distinguish which of the answers was coming from the human and which from the computer, the computer was thought to have matched human intelligence. This experiment was called the Turing test.
The short answer for the definition of a sentient being is that "sentient beings are those who are capable of experiencing suffering (Dukkha)". In the same way that the Turing test is a machine intelligence test, one can develop a sentience test based on the ability of a being to experience suffering. For example, when a hunter shoots at a rabbit, it runs away because its instincts detect a threat to its life. The rabbit does not want to be killed. It wants to avoid the suffering and death caused by a gunshot wound. This is the reason not only human beings are considered sentient beings, because animals also experience Dukkha. Plants are living beings, but not sentient because they don't experience Dukkha. Sentience is also the source of our liberation, because in order to avoid suffering we start studying and practicing Buddhism, wanting to reach liberation as soon as possible.
This test of sentience is interesting if it is related to the concept of life on other planets (I am a big Star Trek fan). If someday we were to meet Vulcans or Klingons and observe them suffering, then they would be considered to be sentient beings, and they might be interested in studying Buddhism and obtaining liberation from this suffering.
A Tibetan teaching says that sentient beings are all beings that have mind and mind is found in all beings that breathe.