Sentience is a weird word. It both refers to the ability to sense something, but also the ability to have consciousness, self-awareness. And it seems self-awareness in a particularly human way. Thus, if someone says of an alien being, "Don't kill it, it's sentient" they do not mean, "It has the capacity for pleasure and pain." Rather, they are claiming it is intelligent, to be included as part of the human community.
Now, obviously non-human animals are (for the most part) sentient in both senses. But I am curious how a word that means responsive to pain and pleasure comes into a metaphysical term for the capacity of humanness? (If anyone has access to the OED and wants to see if it has an answer, I'd appreciate it).
Also, the linguistic confusion frequently makes my life harder.
UPDATE: I ask, and Peter delivers. Click here for the OED entry of sentient.