One argument I've often heard in favor of vegetarianism is that we don't have to kill animals in order to survive. What if we, for biological reasons, were forced to eat other animals? If we couldn't digest plant matter, it would seem we wouldn't have a choice. By the logic of the argument, wouldn't that mean it would be less ethically problematic to kill other animals in order to feed?

But I think you have what philosophers call the "dialectic" of the argument here somewhat backwards. I take it that the argument for vegetarianism is suppose to be something like this: (i) The lives of animals are of moral significance, which is to say that one cannot permissibly kill an animal without good reason; (ii) The need to eat would constitute good reason, but (iii) as a matter of empirical fact, most of us, at least in developed countries, do not need to kill animals to eat, so we do not have such reason; (iv) Mere preference for animal flesh over plant-based foods does not amount to sufficient reason to kill an animal; (v) So we fortunate people living in developed countries ought not to kill animals for food. So the argument is not really that we do not need to eat animals to survive.

It should be clear that the argument does indeed grant that, if one has to kill other animals in order to survive, then that would be morally permissible. But even so, this does not mean that killing those animals has no moral significance under such circumstances. It is simply that other moral considerations are in play. And it does seem that how bad it is to kill an animal depends in some way upon one's reasons for doing so and, indeed, upon how one does so. But none of that seems to undermine the argument.

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