As for the marginal cases, I semi-agree that the logic is sound. It uses individuals to make statements about how entire species should behave towards each other, which I find is going out on a limb. It compares the extremes of one species - e.g. an extremely intelligent pig, to someone with poor ability to reason, for instance, then makes a generalization that, on the basis of these two extreme examples, the entire species needs to behave a certain way. I don't think that is tenable if you are going to use this argument as a guide to action. In real life, the players are the entire population, with normal variation, not simply the highest and lowest 0.5% of each population. So to be truly sound and consistent, the argument should be made either using individuals and only applied to individuals, or using populations and applied to entire populations. It may make sense on paper, to some people, but it fails, in my opinion, when you try to apply it to biological systems. To be blunt, as a scientist, I honestly don't give a rat's smooth bottom if it's right on paper but we can't use it to solve real-life issues. That's why I think it's a poor argument.
RE: In defence of meat: Reflections on society, ethics, experience, and current scientific knowledge – Part II