It's pretty much universally accepted that healthy humans have sentience, but that's where the agreement stops. Many would say that complex animals have sentience, such as dogs, cats, cows, sheep, etc. But what about a mouse, an ant, or a bacterium? And what about a severely brain-damaged human that doesn't seem to be self-aware, and yet is conscious? Or, what about a developing embryo—does it go from being non-sentient to sentient at some point?
This is a significant issue for moral philosophers to address, and one that has plagued them for ages. It is also a hotly debated issue in the realm of animal rights, where sentience is commonly cited as the justification for exempting an animal from undergoing unnecessary suffering. Consider, for example, this passage from the English philosopher Jeremy Bentham:
But a full-grown horse or dog is beyond comparison a more rational, as well as a more conversable animal, than an infant of a day, or a week, or even a month, old. But suppose the case were otherwise, what would it avail? The question is not Can they reason? nor, Can they talk? but, Can they suffer?
So, my question is at what point does it make sense to say that a creature has 'gained sentience', and how do we measure it? And furthermore, once a being has gained sentience, can they lose it if they become severely brain damaged?
A couple of possible criteria come to mind: number of brain cells, the ability to experience emotion, self-awareness (the ability to recognize oneself). Are any of these independently sufficient? Or are there multiple criteria required to declare sentience? Are there other important concerns?