One of my favorite topics-- that of so-called "animal rights"-- came up during this session, and I wasn't entirely pleased with Stefan's answer. I don't want to belabor the point-- since regular readers will know I've written about this topic many times before, even offering up for mental digestion a bullet or two in my summary post-- but I feel compelled to wrap a better bow on the issue than did Molyneux.
I look at the world through the lense of "natural rights", not the "non-aggression principle". Non-human animals and plant life don't have the ability to formulate or will maxims, so the body of natural law-- both on the "accountability" and "granting of self-defense rights" ends-- does not apply to them. Molyneux gets this part right, offering up a similar line of reasoning, but adapted to the "non-aggression principle".
Where he goes wrong is that he fails to articulate that this is a one hundred percent, absolutely pure question about categories: humans have the capacity for reason-- to formulate and will maxims-- whereas non-human animals and plants do not. The questioner poses, "what if a plague reduced all human intelligence to a low level"-- then yes, absolutely "natural rights" would no longer apply to humans either! Because humans would no longer belong to the category of rational beings.
For some reason Molyneux didn't want or think to answer this inquiry with its totally obvious answer, which is why I wasn't satisfied with his treatment of the issue.
On the concept of "outliers", a person with Down Syndrome, or someone who is drunk or asleep, still falls under the "natural rights" umbrella, in the sense that they are a part of the category "human". Similarly, an unusually intelligent ape does not. However, taxonomical categories are not "forever"-- indeed, humans were not always rational beings. And perhaps some day in a million odd years, the category of "dolphin" or something along those lines will need to be re-evaluated, once the intelligent "outliers" are no longer "outliers".
Of course, I recognize that all of this is lofty and abstract, and that in the real world things are complicated, messy, and often unclear. That's why murder trials often have hours or even days of jury deliberation, to determine not only whether murder occurred, but to what degree-- none of which obviates the abstract principles underpinning the concept of murder.
Applied then to the case of this "animal rights" issue, if a person with Down Syndrome commits a brutal murder in the messy real world, the specifics of that case-- did the perpetrator even understand, and to how far of a degree, what they'd done?-- would need to be examined. Similarly, if a super-human intelligence-level monkey, with an "Ape Escape" Spector-like brainiac helmet suddenly appears and commits human genocide, then the specifics of culpability in that case too would need to be analyzed.
And I'm not merely "weaselling out" through a logical consistency escape hatch here: in all morality, classifications and generalizations need to be used to establish guiding principles-- but in the end game called "real life", necessity dictates that it's individual organisms and circumstances which need to be acted upon.