Generally, in law, a sentient being is one with the faculty of sensation and the power to to perceive, reason and think.1
In her 2008 article in the Journal of Animal Law, veterinarian Annamaria Passantino wrote:
"A sentient being is a being that, by virtue of its characteristics, has the capability of experiencing suffering, both at physical and psychological levels, regardless of the species to which it belongs.
"Only the members of the animal kingdom can be sentient, although not every animal species possesses the characteristics that would make their members be considered sentient beings.
"Sentient animals are beings that have a physical and psychological sensibility, which allows them - in the same way as humans - to experience pain and pleasure. And it is certain that they naturally seek, by all means available to them, to avoid painful experiences."
Notice the inclusion of human being in the definition of animal kingdom. In some judicial decisions, the term sentient being has been described, simply, as living beings that feel pain.2
In her 2011 book on animal law, Leslie Bisgould proposed this definition of sentience:
"A conscious being’s capacity to feel and perceive; in reference to animals, it often refers to the specific ability to feel pain and suffer."
The concept of a sentient being is hotly debated in the animal law community where the line between law and philosophy (if there is one), is not clearly drawn. It can give rise to some proposals that might raise the eyebrows of some lawyers.
For example, in a 2007 article published in the Journal of Animal Law by a non-legally-trained writer:
"In 2004, Dunayer states that a someone is a sentient, thinking, feeling individual with unique life experiences whereas a something is not. She rightly criticizes speciesists3 for characterizing non-human animals as things.... I (object) to categorizing nonhuman animals as things ... every sentient being is a someone, not a something....
"No sentient being is an it, that, or thing. Each is equally someone....
"It should be illegal for any human to breed any nonhuman...."
Note Article 13 of the Treaty of Lisbon:
"... animals are sentient beings...."
Although not recognized in law, the views of some animal legal rights expert may have been expressed in these words:
"Fish are not swimming vegetables.... Martine biologists assure us that fish do indeed feel pain (and) form complex social relationships... Fish can count and tell time, they are fast learners, they think ahead and they have distinct personalities."4