Williams was also an unabashed speciesist, and wrote a piece called "The Human Prejudice." which I discovered in lecture format on YouTube. He makes many confusing points in this lecture, so I hope to spend a few posts deconstructing his main claims. As is often the case in his work, it's not at all obvious the route his argument is supposed to be taking. It's best to sit back, take in what he has to say, and try to figure out where you've ended up.
One of the clearest points he makes in these clips is at the very end, during a question and answer period. He says:
"There is a kind of potential contradiction in some lines of animal right thought. On the one hand they say, 'We've got to remember we're part of nature. We share the world with other species. We're one species among others. We weren't sent here as dominators of the world. Second, we're totally different from the other animals because we have moral consciousness. And therefore we can make ourselves into vegetarians.' Now there's an inherent tension between those two things."He's claiming that we can't have it both ways. We can't say we have this deep connection with other animals, because if we really are so relevantly like other animals, than it must be permissible for us to be predators like them.
Which is, on the face of it, just wrong. There is really no tension in the two different kinds of claims that Williams takes the animal rights theorist to be making. I shouldn't be too harsh, because these are off-the-cuff remarks, but he really is most clear about what he thinks in this section of the talk. And the fact is that these two kinds of claims can be perfectly complementary.
First, you might claim that we're relevantly like other animals in that they can think, feel, love, suffer, rejoice, etc., in countless ways, just as we can. Second, you can claim that we are unlike the other animals, because we can reflect on this first fact in the following way. The fundamental way of experiencing the world that we share with animals makes them worthy of consideration in our moral sphere. As moral agents, we have obligations to treat individuals in certain ways. Some of these are individual with whom we share many qualities, but who do not share out moral agency. Where, in any of these thoughts, is there a contradiction?
Animal rights theorist might plausibly claim that most humans go wrong in two ways. First, they see animals as unlike ourselves, in the morally relevant ways, and thus as things to be used for our purposes. Second, they see themselves as animals who are permitted to live up to their carnivorous nature (Williams comes very close to making such a claim.) Again, these claims aren't in contradiction. This time, these claims just happen to be false.
Now Williams hedges in his remarks, by claiming that there is only "a kind of potential contradiction," probably because he realized there was nothing actually contradictory in what he was discussing. All the animal rights theorist is claiming is that we are more akin to animals than we usually suppose in some ways, and in other ways we're less akin to animals. That, of course, isn't the entire argument, but it's perfectly coherent.