Dear Professor Singer:
To begin with an area in which we agree, if it is true that alternative methods of producing meat would substantially reduce animal suffering at trivial cost, I am for adopting them, because I like animals and therefore don't want to see them suffer gratuitously. But I would not be inclined to force my view on others who disagree. I don't see either the necessity or the justification for coercion. If enough people come to feel the sufferings of these animals as their own, public opinion and consumer preference will induce the producers to change their methods. In just the same way, the more altruistic American people become toward foreigners, the greater the costs they'll be willing to incur for the benefit of foreigners.
Where I disagree with you most profoundly is over the question of whether ethical argument either can or should affect how we feel about animals (or foreigners). My view is that ethical argument is and should be powerless against tenacious moral instincts. You point out quite rightly that such instincts often are mistaken. But then the need is for pointing out the mistakes. In the 18th century it was a capital offense in some American states for a human being to have sexual intercourse with an animal. One of the grounds for this harsh punishment was a belief that such intercourse led to the birth of monsters. The belief was unsound, and showing that it was unsound undermined the case for punishment.
To the extent that lack of consideration for animal suffering is rooted in factual errors, pointing out those errors can change our intuitions concerning the consideration that we owe animals. Descartes believed that animals felt no pain—that the outward expressions which we took to reveal pain were deceptive. People who believed this would have no truck with laws forbidding cruelty to animals. We now have good reason to believe that Descartes was mistaken. We likewise have good reason to believe that the Aztecs were mistaken about the efficacy of human sacrifice and that Nazi ideology, like other racist ideologies, rested on misconceptions about evolutionary and racial biology. To accept Cartesian, or Aztec, or Nazi premises and argue merely against the inferences from them would be futile.
To me the most important and worthwhile part of your influential book Animal Liberation is the information it conveys (partly by photographs) about the actual suffering of animals. This information is a valuable corrective to unsound and ignorant thinking. But arguments that do not identify factual errors that underlie or buttress our moral instincts do nothing to undermine those instincts; nor should they. I contended in my reply to your first statement that it is wrong to give as much weight to a dog's pain as to an infant's pain, and that it is wrong to kill one person to save 101 chimpanzees even if a human life is only 100 times as valuable as a chimpanzee's life. I rested these judgments on intuition. Against this intuition you have no factual reply, as you would if my intuition were founded on a belief that dogs feel no pain and that chimpanzees have no mentation.
I said too that the logic of your argument, again at war with our deepest intuitions, was to prefer the life of a dog to the life of a person with Alzheimer's disease. You reply that we should consider the feelings of a person who, knowing that he may someday have the disease, is distressed at the thought of being killed when the disease progresses to a certain point. Fair enough; but what about people, and there are many, who would like to be able to sign an enforceable contract to be killed if they become demented? Such a contract would be unenforceable, and the physician who honored it by killing the Alzheimer's patient would be a murderer. The moral intuition that powers this result may be vulnerable to factual challenge as we learn more about Alzheimer's, as more people suffer from it, and as people come to accept more than they do today the role of physicians as "angels of mercy." What the moral intuition is not vulnerable to is an ethical argument that makes the issue contingent on a comparison of human and canine mental abilities.
I would like, finally, to correct any impression I may have given that I am seeking to justify "giving preference to the suffering of humans, just because it is humans, and not other animals, who are suffering." I don't see myself as engaged, in our debate, in justifying anything except skepticism about the power of philosophical arguments. I do not say that our preferring human beings to other animals is justified—only that it is a fact deeply rooted in our current thinking and feeling, a fact based on beliefs that can change, but not a fact that can be shaken by philosophy. I particularly do not mean to say that we are justified in giving preference to the suffering of humans "just because it is humans" who are suffering. It is because we are humans that we put humans first. If we were cats we would put cats first, regardless of what philosophers might tell us.
Richard A. Posner