Similarity with our own physical constitution? Similarity of behavior when we are in pain and they are in pain? And all the other cues that tell us that, indeed, they are sentient beings?
Ever watched someone work on a chickens feet that have a staph infection? Seems like the infection must hurt like hell because chickens won’t stand on it and limp around. Watching someone treat these infections you’d think chickens pretty much don’t feel pain. I don’t think your suggestion is as clear-cut as you think it is.
