Greg Mankiw remembers the late Richard Rorty who once posed this question to his philosophy class:
Aliens from another planet, with vastly superior intelligence to humans, land on earth in order to consume humans as food. What argument could you make to convince the aliens not to eat us that would not also apply to our consumption of beef?
I can think of a few responses:
1. Unlike cows, humans can make some attempt at self-defense, even against vastly more intelligent aliens. We could likely, at the least, find a way to poison ourselves so as to make our meat unpalatable to alien tastes.
2. Unlike cows, humans can express a desire not to be eaten. Even if lesser in intelligence than the hypothetical aliens, we are capable of communication with them and self-aware enough to object to being consumed.
3. Unlike cows, humans have families, friends, and other sorts of emotional connections. These connections may have analogues among the aliens and could be used to gain their sympathy.
4. Unlike cows, humans are intelligent and creative beings who may have more to offer the aliens than our meat. It is possible that we could enter a mutually beneficial economic arrangement that is valuable to the alien culture and could be used as leverage to prevent our consumption.
(HT: Marginal Revolution.)