In this essay, Korsgaard begins by noting the unstable attitudes that we have towards animals. On the one hand, we seem to agree that it is wrong to inflict pain on an animal or to kill an animal without good reason. On the other hand, it seems like any reason other than mere enjoyment is a good reason to harm or kill an animal. Korsgaard notes that generally, those who want to argue for better treatment of animals will often emphasize the similarities between humans and non-humans whereas those who want to defend the status quo will emphasize the difference between humans and non-humans. Korsgaard will follow neither of these tactics. Instead, she thinks that there is a big difference between humans and non-humans and that it is because of this difference that we ought to treat animals better.
What is the difference between humans and non-human animals? Humans have a capacity for reflective self-awareness. We don't merely act on instinct--we think about our actions and have the capacity to choose to act other than our instincts drive us. Although Korsgaard seems to be open to the possibility that some non-humans have a rudimentary level of self-awareness, humans seem to be rationally and reflexively aware of their own consciousness in a way that other animals are not. Hume seems to think that this difference means that we have no obligations to animals whatsoever. Kant thinks that although we have no obligations to animals, we should treat animals well as a duty to ourselves and other rational beings. In short, Kant thinks that to ignore the suffering of animals is to dull our capacity for sympathy and empathy. To treat animals well is a sort of practice to treat humans well. Because animals are analagous to humans in some ways, we should treat animals well in order to keep up our capacity to care about other people.