After reading Rawls' Justice as Fairness the biggest thing that hit me was the lack of any talk about non-human animals and where they stood in terms of political justice. He made it pretty clear, by omission, that are given no intrinsic value but only were of derivative value, that is, they are only valuable insofar as they are valuable to humans. As far as I'm concerned this sort of anthropocentric view is totally unsupportable. There are a couple reasons for this.
First, If viewed only as of derivative value then all sorts of abuses and poor treatment can be justified. What is wrong with the horrendous conditions of factory farming if the animals treated as such have no intrinsic value? Ultimately this would lead to the extinction of nearly all, if not all, non-human species of animals, as one by their simple existence got in the way of some human right of free travel, or the human good of eating meat.
Second, where do we draw the line between those who are protected and those who are not protected by society? It's easy to say that humans should come first, but lacking a religious argument there is little support for that position. One could argue that it is human rationality that sets us apart from animals, and other forms of life, but I'd imagine that there are developmentally disabled people out there of lower intelligence than some exceptionally intelligent animals, or there is the possibility thereof. But does that mean that it's okay to kill and eat those persons? Or do we simply define the level of protection as covering any animal which has a certain level of intelligence? This too has problems. It is essentially an ad hoc solution that is based on an ideal of anthropocentric thought, wherein we, humans, are the standard by which all other animals are judged. But the problem is that the vast majority, if not all, animals are of such intelligence that they cannot be allowed political rights to the extent that humans enjoy them.
What this boils down to is essentially that we have a species we call human (Homo Sapien) and yet we have no conclusive way to pick every individual humans out as being different from non-humans, except by our intuitions. In evolutionary biology it is a difficult thing to delineate what a species really is. Many of the definitions of species end up making delineating basically arbitrary. For example, the use of genetic markers for defining species does not rely on some innate aspect of a species, it relies on humans saying "you get to be in this species if you have this DNA."
Why is this important? Well, we're going to figure out what exactly we mean by human, and who gets to be human before we can talk about justice.