A reader points to the famous (well . . . famous to the three libertarian vegetarians I know, anyway) Robert Nozick meditation on our obligations towards animals from Anarchy, State and Utopia:
We might try looking at comparable cases, extending whatever judgments we make on those cases to the one before us. For example, we might look at the case of hunting, where I assume that it's not all right to hunt and kill animals merely for the fun of it. Is hunting a special case, because its object and what provides the fun is the chasing and maiming and death of animals? Suppose then that I enjoy swinging a baseball bat. It happens that in front of the only place to swing it stands a cow. Swinging the bat unfortunately would involve smashing the cow's head. But I wouldn't get fun from doing that; the pleasure comes from exercising my muscles, swinging well, and so on. It's unfortunate that as a side effect (not a means) of my doing this, the animal's skull gets smashed. To be sure, I could forego swinging the bat, and instead bend down and touch my toes or do some other exercise. But this wouldn't be as enjoyable as swinging the bat; I won't get as much fun, pleasure, or delight out of it. So the question is: would it be all right for me to swing the bat in order to get the extra pleasure of swinging it as compared to the best available alternative activity that does not involve harming the animal? Suppose that it is not merely a question of foregoing today's special pleasures of bat swinging; suppose that each day the same situation arises with a different animal. Is there some principle that would allow killing and eating animals for the additional pleasure this brings, yet would not allow swinging the bat for the extra pleasure it brings? What could that principle be like? (Is this a better parallel to eating meat? The animal is killed to get a bone out of which to make the best sort of bat to use; bats made out of other material don't give quite the same pleasure. Is it all right to kill the animal to obtain the extra pleasure that using a bat made out of its bone would bring? Would it be morally more permissible if you could hire someone to do the killing for you?)
Such examples and questions might help someone to see what sore of line he wishes to draw, what sort of position he wishes to take. They face, however, the usual limitations of consistency arguments; they do not say, once a conflict is shown, which view to change. After failing to devise a principle to distinguish swinging the bat from killing and eating an animal, you might decide that it's really all right, after all, to swing the bat. Furthermore, such appeal to similar cases does not greatly help us to assign precise moral weight to different sorts of animals.