As so often, the Benthamites could join hands with the evangelicals. "The French have already discovered," Bentham wrote, "that the blackness of the skin is no reason why a human being should be abandoned without redress to the caprice of a tormentor. It may come one day to be recognised that the number of legs . . . or the termination of the os sacrum are reasons equally insufficient for abandoning a sensitive being to the same plight." Observe the transition from slave to animal. Bentham's Utilitarianism looks not to the rationality of the agent or the patient, in the Stoic manner, but to the effect of the agent's actions on all sentient beings, who are from this point of view to be accounted equal. If all pain is evil, as Bentham thought, then the pain of animals—assuming only that they can feel pain—ought not to be ignored in man's moral decisions. The pains of animals might be less, as not including the pains of anticipation, than the pains felt by man, but that is no reason for not taking them into account. "The question is not," so Bentham argues, "Can they reason?" nor "Can they talk?" but "Can they suffer?" So whereas Plutarch and Porphyry thought it necessary to begin their case against treating animals merely as chattels by arguing that animals have a share in reason, for Bentham it is irrelevant whether or not they are rational and to what degree. It is enough that they are capable of suffering.
In his later writings, however, Bentham reverted to something more like the Aquinas-Kant position. The Traités edited by Dumont condemn cruelty to animals only—if Dumont can be trusted—on the ground that it can give rise to indifference to human suffering. In his Constitutional Code, Bentham's emphasis is not on suffering but on the alleged fact, made secondary in the Principles, that mature quadrupeds are more moral and more intelligent than young bipeds. I do not know why Bentham changed his mind. But perhaps he boggled, and not unnaturally, at the conclusion that to determine whether an act is right we ought to take into consideration its consequences for every sentient being.
(John Passmore, "The Treatment of Animals," Journal of the History of Ideas 36 [April-June 1975]: 195-218, at 211 [italics in original; footnotes omitted])