Singer's supreme principle is that all sentient beings are entitled to equal consideration of their interests. A being has interests if it is capable of suffering and enjoyment. This capacity is a prerequisite for having interests at all, and the actual interests that a being has are determined by the particular kinds and degrees of suffering and enjoyment of which it is capable. Equal interests must be equally respected, without regard to the species of the creatures whose interests they are. One may treat two creatures differently because one is less sensitive than the other to some kind of suffering, but two equally sensitive creatures may not be treated differently merely because they belong to different species. If my dog and I both have headaches then the dog should have the one available aspirin if it has the worse headache. To treat the dog's pain as less important because it is a dog not a man is speciesism (a nasty word for a nasty thing).
(John Benson, "Duty and the Beast," Philosophy 53 [October 1978]: 529-49, at 530)