“Whenever an animal is forced into the service of man, every one of us must be concerned for any suffering it bears on that account. No one of us may permit any preventable pain to be inflicted, even though the responsibility for that pain may not be ours. No one may appease his conscience by thinking that he would be interfering in something that does not concern him. No one may shut his eyes and think the pain, which is therefore not visible to him, is nonexistent.”