The argument from evil, with special reference to animal pain, is an increasingly fashionable objection to the existence of God. Since this is typically deployed against Christian theism, that way of framing the debate creates the misimpression that secular thinkers have a monolithic view of animal rights and–what is more–that this is the default view of animal rights which everyone would normally share, but Christians, are forced by their theological precommitments to deny the obvious.
This is the view of animal pain and animal rights popularized by atheists like William Rowe and Peter Singer. I’m relating the concepts of animal pain and animal rights since, unless you think animals have a right not to suffer pain, animal pain is not a moral issue.
For now I’m not discussing my own position on animal rights. I’m simply wish to draw attention to a tendentious presupposition of the argument. I wish to make the point that secular thinkers do not have a monolithic view of animal rights.
And to that extent, an atheist can’t presume a certain view of animal rights when he is mounting the argument from evil. For this isn’t simply a point of disagreement between believers and unbelievers. Rather, there are deep-seated differences of opinion within the secular camp on this key presupposition.
Take the debate between Peter Singer and Richard Posner. Both men are secularists. But they don’t share the same view of animal rights. Here is some of what Posner has to say:
I do not agree that we have a duty to (the other) animals that arises from their being the equal members of a community composed of all those creatures in the universe that can feel pain, and that it is merely "prejudice" in a disreputable sense akin to racial prejudice or sexism that makes us "discriminate" in favor of our own species. You assume the existence of the universe-wide community of pain and demand reasons why the boundary of our concern should be drawn any more narrowly. I start from the bottom up, with the brute fact that we, like other animals, prefer our own—our own family, the "pack" that we happen to run with (being a social animal), and the larger sodalities constructed on the model of the smaller ones, of which the largest for most of us is our nation. Americans have distinctly less feeling for the pains and pleasures of foreigners than of other Americans and even less for most of the nonhuman animals that we share the world with.
Now you may reply that these are just facts about human nature; that they have no normative significance. But they do. Suppose a dog menaced a human infant and the only way to prevent the dog from biting the infant was to inflict severe pain on the dog—more pain, in fact, than the bite would inflict on the infant. You would have to say, let the dog bite (for "if an animal feels pain, the pain matters as much as it does when a human feels pain," provided the pain is as great). But any normal person (and not merely the infant's parents!), including a philosopher when he is not self-consciously engaged in philosophizing, would say that it would be monstrous to spare the dog, even though to do so would minimize the sum of pain in the world.
I do not feel obliged to defend this reaction; it is a moral intuition deeper than any reason that could be given for it and impervious to any reason that you or anyone could give against it. Membership in the human species is not a "morally irrelevant fact," as the race and sex of human beings has come to seem. If the moral irrelevance of humanity is what philosophy teaches, and so we have to choose between philosophy and the intuition that says that membership in the human species is morally relevant, then it is philosophy that will have to go.
Toward the end of your statement you distinguish between pain and death and you acknowledge that the mental abilities of human beings may make their lives more valuable than those of animals. But this argument too is at war with our deepest intuitions. It implies that the life of a chimpanzee is more valuable than the life of a human being who, because he is profoundly retarded (though not comatose), has less mental ability than the chimpanzee. There are undoubtedly such cases. Indeed, there are people in the last stages of Alzheimer's disease who, though conscious, have less mentation than a dog. But killing such a person would be murder, while it is no crime at all to have a veterinarian kill one's pet dog because it has become incontinent with age. The logic of your position would require treating these killings alike. And if, for example, we could agree that although a normal human being's life is more valuable than a normal chimpanzee's life, it is only 100 times more valuable, you would have to concede than if a person had to choose between killing one human being and 101 chimpanzees, he should kill the human being. Against the deep revulsion that such results engender the concept of a transhuman community of sufferers beats its tinsel wings ineffectually.
You say that some readers of Animal Liberation have been persuaded by the ethical arguments in the book, and not just by the facts and the pictures. But if so, it is probably so only because these readers do not realize the radicalism of the ethical vision that powers your view on animals, an ethical vision that finds greater value in a healthy pig than in a profoundly retarded child, that commands inflicting a lesser pain on a human being to avert a greater pain to a dog, and that, provided only that a chimpanzee has 1 percent of the mental ability of a normal human being, would require the sacrifice of the human being to save 101 chimpanzees. If Animal Liberation had emphasized these implications of your utilitarian philosophy, it would have had many fewer persuaded readers; and likewise if it had sought merely to persuade our rational faculty, and not to stir our empathetic regard for animals.