Richard Rorty

From Philosophy and the Mirror of Nature (paraphrased)

Personhood is a matter of decision rather than knowledge, an acceptance of another being into fellowship rather than a recognition of a common essence.

Knowledge of what pain is like or what red is like is attributed to beings on the basis of their potential membership in the community. Thus babies and the more attractive sorts of animal are credited with “having feelings” rather than (like machines or spiders) “merely responding to stimuli.” 

To say that babies know what heat is like, but not what the motion of molecules is like is just to say that we can fairly readily imagine them opening their mouths and remarking on the former, but not the latter. To say that a gadget that says “red” appropriately doesn't know what red is like is to say that we cannot readily imagine continuing a conversation with the gadget....

Attribution of pre-linguistic awareness is merely a courtesy extended to potential or imagined fellow-speakers of our language. Moral prohibitions against hurting babies and the better-looking sorts of animals are not based on their possessions of feeling. It is, if anything, the other way around....

Rationality about denying civil rights to morons or fetuses or aboriginal tribes or blacks or Martians or trees is a myth. The emotions we have toward borderline cases depend on the liveliness of our imagination, and conversely.