Many people think that the very idea of animal rights is implausible. Suggesting that animals are neither rational nor self-aware. Immanuel Kant thought of animals as "man´s instruments", deserving protection only to help human beings in their relation to one another: "He who is cruel to animals become hard also in his dealings with men." Jeremy Bentham took a quite different approach, suggesting that mistreatment of animals was akin to racial discrimination: "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 of the skin is no reason why a human being should be abandoned without redress to the caprice of a tormentor... 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 the case were otherwise, what would it avail? The question is not, Can they reason? nor, Can they talk? but, Can they suffer?"

John Stuart Mill concurred the analogy to slavery.

(Taken from the book: Animal Rights: Current Debates, from the Introduction: What Are Animal Rights by Cass R. Sunstein, Oxford University Press, 2004, NY)