Christine M. Korsgaard in The Point:
What sorts of philosophical problems do we face because of the existence of non-human animals? Most humane people would agree that their existence presents us with some moral and legal quandaries. And recently, but only recently, philosophers have taken a serious interest in the character of animal minds. But I have come to think that animals present us with a philosophical problem deeper than either of those—that the existence of non-human animals is the source of a profound disturbance in the way that human beings conceptualize the world. It is almost as if we—I’m using “we” to mean “us human beings” here—are unable to get them firmly into view, to see them for what they really are.
Many people, to take one small example, find nothing odd about the sentence, “I live alone with a cat.” Okay, granted, someone might also say, “I live alone with a child,” at least so long as the child was a very small one. But “I live alone with four children” would be starting to put the language under stress, even if they were all toddlers, while “I live alone with four cats” would not. Here’s another example: People wondering about whether there might be life on other planets sometimes ask, “Are we alone in the universe?” Just look around! Well, you may reply, they mean to ask whether there is any other intelligent life in the universe. Right. Just look around!
Animals also seem to pop in and out of our moral view. Most people would agree that it is wrong to hurt or kill a non-human animal without a good reason, but then it turns out that any reason, short of malicious pleasure, is reason enough.