A rough guide to what we owe to animals is this: we owe to them whatever kind of treatment we believe the severely retarded would be owed in virtue of their intrinsic natures by morally sensitive Martians. (p. 227)Okay. If I want to know what I owe to cats and dogs, I just have to consult my convictions about those morally sensitive Martians, and what they owe to the severely retarded.
It sounds absurd, but on second thought, maybe this is not simply a case of "ignotum per ignotius"--trying to figure out the unknown by pondering the even more unknown.
We have a complex set of predispositions when it comes to the treatment of animals. Some animals are elite, and not to be tampered with. Some can be killed for food or even just for fun. All that is so deeply entrenched that it's hard to think clearly about what's really ethical or unethical.
Enter, the aliens. What would they owe to us? Another fresh start on the animals problem: What would we owe to aliens? It was with that honorable question in mind that I went to the movie District 9 a couple of days ago. I'm happy to say that about 15 minutes of the movie did touch on deep matters of ethics. The rest was ultra-gross-out mayhem of the first order, with a very light helping of another philosophical issue--the problem of personal identity. (Think Jeff Goldblum and The Fly.)
The aliens in the movie are treated in a way that's a little odd. They're not dismissed as "just animals," even though they look a lot like large (humanoid) insects. Rather, they're treated like a second-class human minority group--like black South Africans under apartheid.
The difficult question lurking in the background: what would we owe to an alien species that landed on earth? What criteria would we use to decide what their status was? I suspect if we thought these things through, we really would be in a better position to know what we owe to animals.
NB: There's more to the McMahan's quote. He asks us to think about what morally sensitive Martians would owe to severely retarded humans. Is that really the right question to ask, if we're worried about what we owe to animals? I very much doubt it, but that's another story.