As for practical criteria to define what constitutes a sentient being (or “living being” in the language of the suttas) – specifically referring to humans and animals – before I present my view on the matter, I want to recall the sequence of Dependent Co-Arising:
(…) Fabrications (Sankharas) → [Rebirth-linking] Consciousness → Name-Form → Sense-Bases (…)
With this in mind, I understand the subject in the following way:
Humans and animals possess Name-and-Form, or Mentality-Materiality (mental and bodily components).
Form is the body constituted by material elements – the “Four Elements.” Name refers to the mental components – feelings, perceptions, consciousness, and fabrications (Sankhara).
The main bodily component associated with mental activities is the nervous system. Thus, a being with a developed nervous system, or with the necessary kamma to develop one (in the case of early-stage embryos), can be considered a “living being.”
The reason beings with a nervous system are considered sentient is easy to understand. However, in the case of embryos, this may not be obvious to those without direct knowledge of the rebirth process. The Buddha clearly states that the embryo is already a “living being” and that its destruction yields karmic results – it is already a violation of the First Precept, and the death of a human embryo justifies the expulsion of a monk from the Sangha. If we have faith in the Buddha, then we must understand that the embryo is already a living being.
Embryos without a nervous system do not yet have the conditions necessary for feeling, perception, and consciousness – these conditions would be the bases-of-contact. But they have the kamma of the being undergoing rebirth. Since kamma is a type of Sankhara (fabrication) it is possible to say that embryos also possess Name (Mentality). This would be why the destruction of human and animal embryos already has karmic consequences.
Having said this about humans and animals, it is also necessary to talk about other types of beings, which have life in a biological sense but are not sentient.
Beings formed of organic matter but without a nervous system (plants, fungi, and non-animal microorganisms), despite having organic life, are not “alive” in the canonical sense of the word (they are not sentient). Therefore, the death of these beings alone does not yield karmic consequences.
Similarly, the body of a person in brain death, whose nervous system has been irreversibly destroyed, has only organic life but no longer possesses consciousness. The conditions necessary for consciousness in that body were lost when the nervous system was destroyed. For this reason, discontinuing the care of what was once a person, with the sole intention of allowing the natural progression of death, should also not generate karmic consequences for those involved.
To keep it simple: if it has a nervous system, or can develop one, then it is a living being.