"They have no consciousness therefore. Therefore what? Therefore we are free to use them for our own ends? Therefore we are free to kill them? Why? What is so special about the form of consciousness we recognize that makes killing a bearer of it a crime while killing an animal goes unpunished?...all this discussion of consciousness and whether animals have it is just a smokescreen. At bottom we protect our own kind. Thumbs up to human babies, thumbs down to veal calves."

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If a being suffers there can be no moral justification for refusing to take that suffering into consideration. No matter what the nature of the being, the principle of equality requires that its suffering be counted equally with the like suffering - insofar as rough comparisons can be made - of any other being. So the limit of sentience is the only defensible boundary of concern for the interests of others. To mark this boundary by some other characteristic like intelligence or rationality would be to mark it in an arbitrary manner. Why not choose some other characteristic, like skin color?

— Peter Singer