One restriction on the absolutism of man's rule over Nature is now generally accepted: moral philosophers and public opinion agree that it is morally impermissible to be cruel to animals. And by this they mean not only that it is wrong to enjoy torturing animals—which few moralists would ever have wished explicitly to deny, however little emphasis they might have placed on cruelty to animals in their moral teaching—but that it is wrong to cause them to suffer unnecessarily. "The Puritan," Macaulay once wrote with condemnatory intent, "hated bear-baiting, not because it gave pain to the bear, but because it gave pleasure to the spectators." In other words, what they hated—and by no means perversely—was the enjoyment of animal suffering; to the mere fact that the bears suffered as a consequence of human action they were indifferent. That, on the whole, is the Christian tradition. But now the situation has changed; not only cruelty—the enjoyment of animal suffering—but callousness, indifference to animal suffering, not taking it into account in deciding how one ought to act, is morally condemned.
Controversies no doubt remain. But they now turn around the question what is to count as "making animals suffer unnecessarily," whether, for example, vivisection or fox-hunting are, in these terms, morally justifiable. By looking in some detail at the way in which the general moral principle that it is wrong to act callously has gradually won acceptance, we can hope to see revealed, first, how reluctantly Western man has accepted any restriction whatsoever on his supposed right to deal as he pleases with Nature and, secondly, how changes in his moral outlook have nevertheless come about.
(John Passmore, "The Treatment of Animals," Journal of the History of Ideas 36 [April-June 1975]: 195-218, at 195 [italics in original; footnotes omitted])