That is an absurd argument. And the absurdity of that argument is plainly shown on this New York Times article.
The Meat Eaters [New York Times]Our factory farms, which supply most of the meat and eggs consumed in developed societies, inflict a lifetime of misery and torment on our prey, in contrast to the relatively brief agonies endured by the victims of predators in the wild. From the moral perspective, there is nothing that can plausibly be said in defense of this practice. To be entitled to regard ourselves as civilized, we must, like Isaiah’s morally reformed lion, eat straw like the ox, or at least the moral equivalent of straw.
But ought we to go further? Suppose that we could arrange the gradual extinction of carnivorous species, replacing them with new herbivorous ones. Or suppose that we could intervene genetically, so that currently carnivorous species would gradually evolve into herbivorous ones, thereby fulfilling Isaiah’s prophecy. If we could bring about the end of predation by one or the other of these means at little cost to ourselves, ought we to do it?
The basic issue, then, seems to be a conflict between values: prevention of suffering and preservation of animal species. It is relatively uncontroversial that suffering is intrinsically bad for those who experience it, even if occasionally it is also instrumentally good for them, as when it has the purifying, redemptive effects that Dostoyevsky’s characters so often crave. Nor is it controversial that the extinction of an animal species is normally instrumentally bad.
The claim that existing animal species are sacred or irreplaceable is subverted by the moral irrelevance of the criteria for individuating animal species. I am therefore inclined to embrace the heretical conclusion that we have reason to desire the extinction of all carnivorous species[.]
If you missed that, Professor McMahan thinks it would be a good idea to get rid of all carnivorous species in nature, if we can. Few would disagree that this is crazy. But McMahan's position is perfectly logical, as long as one accepts all the assumptions that he makes. And the crucial assumption that logically leads to McMahan's crazy result is this: "It is relatively uncontroversial that suffering is intrinsically bad for those who experience it."
NO! It is very controversial as to whether suffering is intrinsically bad for animals that experience it. In fact, it is only a distinct (but very loud) minority of humans who think that animal suffering is unacceptable. Vast majority of humankind, throughout the globe and throughout history, has always thought that animals were there to be hunted and eaten, with pain necessarily being inflicted in the process.
To be sure, causing more pain than necessary to kill and eat an animal -- for example, gratuitous animal abuse -- has almost always been an object of universal human condemnation. This is the reason why many people who are not advocates for veganism are nonetheless repulsed by the manner in which some dogs are slaughtered in Korea, i.e. being beaten to death. And the Korean also agrees with them: "treating animals with dignity and respect means that the current way in which dogs raised for their meat in Korea must change. The tiny cages must go, and so must the unsanitary living condition for those dogs. The method of slaughtering the dogs must be regulated as well, so that the dogs may end their lives in a humane, dignified manner."
But once you accept the premise that any pain caused on any animal is bad, you are logically compelled to arrive at a crazy result, like the idea that to the extent possible, we should gradually remove the carnivorous species from the Earth -- which likely include ourselves.
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