> If nature is the source of natural rights, then you would expect their violation in nature to be an exception rather than a rule. If it is instead the rule, then what even is the meaning of calling it a "natural right"?

Plato, Aristotle, Aquinas, etc, were working in a very different framework from most people today. They believed that universals were really existent entities – either in some transcendent eternal realm (Plato and Platonists), or somehow immanent in their instances. From those realist assumptions, it makes sense to view a universal (nature/essence/subject/species/etc) as an ideal against which something could be measured. The fact that real life very often fell short of the ideal didn't make it any less ideal. And it is "natural" because it is part of the nature of things. Whereas, most people today are nominalists, and given nominalist assumptions, that whole approach doesn't make much sense. But now we've shifted the philosophical problem from natural rights, to who has the correct metaphysics of universals.

They also viewed teleology as real in a way in which mainstream contemporary thought does not. Purpose was not an exclusive property of conscious beings (humans and animals), it was inherent in everything. Teleological explanations (final causes) were viewed as causal explanations equal in ontological status to efficient causation, whereas Enlightenment and post-Enlightenment thought has (on the whole) redefined causation to mean efficient causation only. Purpose was inherent in the very nature of things, so it makes sense to judge actions based on whether they further those purposes or fight them.

> The problem with all the philosophers you've listed is that they were, essentially, arguing towards a predetermined conclusion, which is that the way they wanted their society to be is also the "natural" way for it to be.

Okay, but if you are going to accuse them of that, why not levy the same accusation against philosophers whose positions are closer to your own? If realism about universals is an example of "motivated reasoning", why isn't irrealism about them equally so? If a broader understanding of causation, in which teleology and efficient causation are ontologically equal, is an example of "motivated reasoning", why isn't Bacon/Hume/etc's position that efficient causation has ontological priority over teleology equally so?

> The very fact that human morality has varied so extremely over different cultures, and even in the same culture over time (which people often underestimate based on pop and folk history, I should add), is yet another piece of evidence that there's nothing "natural" about all this.

That's an old argument: morality can't be objective because people don't agree on what is moral. It also has an old response: if that's true, then fact can't be objective either, because people don't agree on what is factual.

> For the purposes of this conversation, "reason" as a term means what people today, in the society that we live in, understand as such.

It is a vague squishy term without a clear definition. And when we try to be precise about what it means, we find that even today, people disagree about which precise definition is the correct one. And that's important, because different definitions of "reason" can produce different answers to questions such as "can reason confer rights?"

> I'm not interested in sophistry.

Implying that the person you are talking with is engaging in "sophistry" is not particularly civil, nor conducive to thoughtful discussion.