Philosophers have shown that the standard reasons offered to exclude animals from the moral circle, and to justify not assessing our treatment of them by the same moral categories and machinery we use for assessing the treatment of humans, do not meet the test of moral relevance. Such historically sanctified reasons as “animals lack a soul,” “animals do not reason,” “humans are more powerful than animals,” “animals do not have language,” “God said we could do as we wish to animals” have been demonstrated to provide no rational basis for failing to reckon with animal interests in our moral deliberations. For one thing, while the above statements may mark differences between humans and animals, they do not mark morally relevant differences that justify harming animals when we would not similarly harm people. For example, if we justify harming animals on the grounds that we are more powerful than they are, we are essentially affirming “might makes right,” a principle that morality is in large measure created to overcome. By the same token, if we are permitted to harm animals for our benefit because they lack reason, there are no grounds for not extending the same logic to non-rational humans, as we shall shortly see. And while animals may not have the same interests as people, it is evident to commonsense [sic] that they certainly do have interests, the fulfillment and thwarting of which matter to them.
(Bernard E. Rollin, "The Moral Status of Animals and Their Use as Experimental Subjects," chap. 41 in A Companion to Bioethics, 2d ed., ed. Helga Kuhse and Peter Singer [Malden, MA: Wiley-Blackwell, 2009], 495-509, at 497 [italics in original])