Many animal rights and animal welfare activists (such as Peter Singer and Tom Regan) take aim at speciesism, which is the thought that human interests have greater weight than the interests of animals, just because they are the interests of humans. They suggest, in different ways, that many other animals possess the morally relevant characteristics deserving of equal moral consideration. (For example, other animals can experience pleasure and pain.) Thus, to give less basic consideration to the interests (or the sufferings) of animals is unjustifiable.
This dominant dialectic in the animal ethics literature poses a dichotomy of sorts: either you accept anti-speciesism, or you're a speciesist. (And speciesism is just as bad as sexism and racism, so you don't want to be a speciesist.)
Cora Diamond has questioned this dichotomy, most notably in her article, "Eating Meat and Eating People." Recently--while preparing for my class "The Philosophers & the Animals" in the fall--I've been reading some work by Rush Rhees (a student of Wittgenstein's) on humans and animals. It's struck me that Rhees' thoughts help to show why the "either speciesism or anti-speciesism" dichotomy is indeed a false dichotomy.
Rhees points out that there are differences between humans and animals which make a difference in the kind of meaning or significance we can attach to their actions. Rhees' quips: "'He was faced with a very difficult choice.' That is something you never would say of an animal" (167). And in a similar vein: "An otter cannot lead one kind of life rather than another" (169). This is just to point out that animals are not moral agents. Anti-speciesists would agree, but argue that other beings can be harmed, or can suffer, besides moral agents (including infants and the severely cognitively impaired, even among members of the human species). But I suspect that what Rhees is suggesting is that because human life can be understood as having a "course" and as being answerable to moral standards, the ways in which humans can be harmed (as well as the ways in which they can fail themselves) are significantly different from they ways in which animals can be harmed. A person could do something so bad to another person that it might de-rail that person's whole life, but pace Rhees, I wonder whether it makes sense to say that a factory farmed pig's life has been de-railed by its handlers. A pig doesn't "have a life" in the this sense.
But that humans "have a life" in a way that other animals do not, on Rhees' view, doesn't imply anything about the relative value of animal lives, in comparison to human lives. It doesn't make sense to say either that we should give humans and animals equal or unequal consideration, because those claims require making a comparison which Rhees thinks doesn't make any sense.
This is because, on the one hand, we can't reduce our understanding of human life to the way we understand animals. Rhees would say such a reduction would forget the significance of language and culture to the shape of human life. But on the other hand, it doesn't make sense to say things like, "Animals don't have language, or culture, or a soul, and so they are inferior to humans." That animals lack language and culture and a soul just shows that they aren't humans! But for Rhees the judgment that this establishes their "inferiority" makes no sense. (And to this extent, the anti-speciesist is right to draw attention to the fact that animals can experience pleasure and pain, that animals can be abused, etc.)
That is, it doesn't make sense to say we should give equal or unequal consideration to animals lives, because the kind of consideration we give to human life, as opposed in animal life, is different in form. But then if we try to compare humans and animals, to determine whether they have the same or different levels of (moral) importance, Rhees' basic idea is that we're trying to compare apples and oranges. This is a problem for both the speciesist and the anti-speciesist, and suggests, if Rhees is right, that the dominant way of drawing lines in the animal ethics literature rests upon a confusion. But, as with Diamond's work, it is still possible to reject the anti-speciesist program but to push forward with the thought that many of our current practices involving animals are deeply morally problematic.
[References are from Rush Rhees, Moral Questions, edited by DZ Phillips (St. Martin's Press, 1999).]