Why this question matters
If sentience is what makes someone able to be harmed, we need to know who has it. We can't look inside another being's mind. But scientists use three kinds of clues.
Clue 1: the body (the physiological standard)
The main clue is having a centralized nervous system, meaning one with a "command center," like a brain or ganglia.
That's why the text considers these not sentient:
- beings with no nervous system, like bacteria, fungi, plants, and sea sponges;
- beings with scattered nerves but no center, like jellyfish and sea cucumbers.
Other clues in the body help confirm it: having nociceptors (sensors that detect injuries and alert the brain) and producing substances that work as natural painkillers.
Clue 2: behavior
If an animal changes what it does depending on the situation, learns to avoid what hurts it, and looks for what's good for it, that suggests it's having experiences. What matters is looking at behavior as a whole, not at one single reaction.
Clue 3: the logic of evolution
Consciousness seems useful for survival when an animal has to make flexible decisions in a changing environment. Feeling pain helps avoid danger in ways a fixed reflex couldn't. So it makes sense that it appeared in animals with more complex bodies and lives.
In short
- The main clue is having a nervous system with a center, like a brain.
- Flexible behavior and the logic of evolution back up that clue.
- Plants, fungi, and bacteria don't have what it takes to feel.