Suffering, by its nature, is awful, and so one needs an excellent reason to cause it. Occasionally, one will have such a reason. Surgery may cause a human being severe postoperatory pain, but the surgeon may be right to operate if that’s the only way to save the patient.
And what if the sufferer is not a human, but an animal? This doesn’t matter. The underlying principle is that suffering is bad because of what it’s like for the sufferer. Whether the sufferer is a person or a pig or a chicken is irrelevant, just as it’s irrelevant whether the sufferer is white or black or brown. The question is merely how awful the suffering is to the individual.
Stuart Rachels, ‘Vegetarianism’, in T. L. Beauchamp & R. G. Frey (eds.), The Oxford Handbook of Animal Ethics, Oxford, 2012, pp. 883-884
Ethics is founded on evidence that can’t be shared. My experience of severe pain gives me reason to believe that nihilism is false. In other words, when I am in severe pain, that pain, as it’s presented to me, gives me evidence that it’s bad in some way. I can’t share this evidence with you; you can’t feel my pain. Even if you could peer inside my head and see it, you wouldn’t be presented with it in a way that gave you evidence of its badness. But you, of course, are in the same position regarding your pain: when you are in severe pain, that pain, as it’s presented to you, provides you with evidence that it’s bad in some way. So, each of us has evidence for his or her severe pain being bad in some way. In the case of infants and nonhuman animals, the evidence is there, but the creature is too unsophisticated to recognize it as such.
Stuart Rachels, Hedonic Value, Ph.D. dissertation, University of Syracuse, 1998, p. 35