Some people1 when hearing someone argue for the rights of some being (a fetus, a dog, a negro) to which they would not grant moral status or rights assume that the reason the thing is being granted rights is because it's life and thus query if this rights then apply to plants or bacteria. To respond to this (valid) criticism, you must draw a non-arbitrary line for what has rights and what doesn't. Immanuel Kant chose rationality (which is itself ambiguous and oftentimes unhelpful. A dog? An infant? A Jerry Springer guest?). John Locke, among others, chose "human" (Again, often unhelpful. A fetus? An embryo?)
My preferred distinction, and the one usually most favourable to people granting rights to questionable entities such as fetuses or dogs, is the ability to suffer. (Still ambiguous, perhaps, but to a lesser extent.)
The day may come when the rest of the animal creation may acquire those rights which never could have been withholden from them but by the hand of tyranny. The French have already discovered that the blackness on the skin is no reason why a human being should be abandoned without redress to the caprice of a tormentor. It may one day come to be recognised that the number of legs, the villosity of the skin, or the termination of the os sacrum are reasons equally insufficient for abandoning a sensitive being to the same fate. What else is it that should trace the insuperable line? Is it the faculty of reason, or perhaps the faculty of discourse? 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 they were otherwise, what would avail? The question is not, Can they reason? nor Can they talk? but Can they suffer?
-- Jeremy Bentham
Such as Saige
, to whom this writeup was originally intended as a reply to, but her writeup is gone now so I'm expanding it to fill in the gaps.