Read Part 1
One of the threads throughout Williams' lecture reflects the deep moral skepticism he was known for. The exact nature of Williams' skepticism is open to debate, though I am quite sympathetic to Parfit's view that it, under inspection, dissolves into nihilism. That is, Williams' attempts to make sense of normative claims and avoids any appeal to true objective facts is futile. Either normative claims are meaningful and true or they're not. Any attempts to water them down into statements about natural facts strips them of any significant meaning.
In the talk, Williams urges us to remember that there is no "cosmic perspective" from which human beings might be thought of as important or unimportant. There is only the human perspective, from which it is obviously important that we are important. So when humans decide to privilege other humans in our moral assessments, and downgrade the value of non-human animals, this is not a judgment of cosmic significance, just of significance to us. Given that we're humans, it's no surprise that we would value humans far above other animals.
There's a clear push to a relativist view here, which is not surprising given Williams' other views. One useful response, as always, is the argument from marginal cases. Like others who have tried this line against the animal rights position, Williams' view leaves wide open the path to obviously vile implications.
But aside from those usual criticisms, it's useful to realize what Williams' view implies for other moral positions that are less controversial than animal rights. For example, he has no defense against someone who wanted to defend the view race-based prejudices could be justified from our human perspective. Perhaps the devaluation of a single race would not make sense from this perspective, but one could easily justify a prejudice in favor of members of any given individual's own racial group. As Williams says about the human prejudice, one could just say that a preference for one's own race is "hardly surprising."
Yet the question should not be, "Is it surprising?" (lots of horrible things are unsurprising) but, "Is it justified?" Williams thinks this cannot really have an answer, because we just have our own limited perspective, and there is no cosmic perspective. However, our moral perspective is precisely the point from which we naturally universalize. That's why a moral preference for one's own race, though perhaps unsurprising, is seen as deeply perverse. We don't think racial criterion can, in fact, justify differences in moral regard. Under sufficiently critical reflection, we should not see our race as playing a role in our moral status, and likewise, the moral status of others.
On the whole, Williams' view is consistent, I believe. His meta-ethical views do, in fact, entail that a truly committed racist has no reason not to be racist. Williams therefore could coherently claim that he had no reason to refrain from using non-human animals as means. So many who might want to join Williams in his conclusion of rejecting animal rights must beware that they do so a great cost. He is able to embrace speciesism because he's not convinced he truly needs to justify any of his ethical beliefs