John Rawls in his A Theory of Justice writes:
Rawls may be certain that cruelty is wrong and that extirpation of a species is an evil, but where on earth does his certainty come from? Exactly why is cruelty to animals or humans evil? Because it's wrong to cause gratuitous pain and suffering? Why is it wrong to do that?
Rawls seems to be asking us to consult some sort of moral sense that is hard-wired into the human species and which condemns such acts, but we might well ask where such a sense comes from and why we should pay it any heed.
There are really only two plausible accounts of the provenience of such a sense. Either the moral sense is instilled in us by an intelligent, omniscient, perfectly good creator, or it is the product of physico-chemical forces which blindly and purposelessly selected them to suit us for life in the environment we found ourselves in during the paleolithic period of human history.
I don't know which of these Rawls would opt for, although I suspect he would choose the latter. Whatever his choice, why should anyone who holds that our moral sense is the product of some purely physical Darwinian process believe that cruelty to animals is evil? Why should they believe that anything at all is evil? If our moral sense is simply an epiphenomenon of our evolutionary history then there is no such thing as evil.
If human moral values are somehow a genetic phenomenon which evolved to assist us in the survival of our species, if that's all they are, then why should anyone feel bound by them? Why should an individual person care about the survival of the species? More to the point, what does the survival of some animal species like the snail darter have to do with the survival of humans?
A reader might reply that an individual should reject all forms of cruelty for the simple reason that he wouldn't want himself to be treated cruelly. It is no doubt true that most people wouldn't want to be treated cruelly, but that is not a reason why the individual in question should not be cruel to others, especially if he can get away with it, and it is certainly not a reason why he shouldn't be cruel to animals.
Rawls' belief that cruelty to animals is evil is intelligible only if our moral sense was "written on our hearts" by God. Otherwise, it is completely illusory and might just as well be treated like some vestigial appendix whose retention is purely optional. Absent a transcendent moral authority, moral judgments such as Rawls makes are utterly meaningless except that they reveal something about the subjective states of the person who makes them. They are not unlike declaiming that it is evil to prefer chocolate ice cream to almond swirl.
It is ironic to read naturalists like Peter Singer, and perhaps John Rawls, condemn the mistreatment of animals while simultaneously rejecting the only plausible basis they could have for making such moral judgments. Naturalists frequently perform a moral piggy-back on theism, tacitly using the assumptions of theism to support their moral claims, while at the same time declaring in vociferous accents that the belief system they're exploiting in order to smuggle in their asseverations that cruelty is wrong, is really just myth and superstition.
Naturalists are like heavy cigarette smokers. They know their habit is killing them but they just can't give it up. Naturalists know that they can't live with the logical entailments of their atheism, but they refuse to quit. Instead, like parasites unable to provide their own moral sustenance, they fasten on to the hide of theism and ride it as long as they can, sucking their nourishment from it. Then, just before they have to admit to what they're doing, they leap off and proceed to boast about what free-thinkers they are and how unnecessary the steed whose life they've been depending upon is. They act as if neither they, nor anyone else, recognizes the metaphysical free-loading they've just been engaged in.
And these folks teach at places like Princeton and Harvard.