In reference to Jude Doughtery's synthesis of Aquinas with modern rights theories:

As much as I respect Dr. Dougherty as a senior, I think he is incorrect in his attempt to harmonize a modern rights theory with Aquinas. I don't think he's modified his views since he wrote that piece (which was in the 80s), but then again, I haven't read through his entire corpus. The tendency of modern Thomists to talk about "metaphysical" bases can be rather confusing, though I can see what he is trying to do, to exact a metaphysical argument from Aquinas's theology, just as someone might try to extract an account of ethics or natural law from it. But nonetheless, I think equality of nature is mostly irrelevant to Aquinas's treatment of justice -- it is important in his explanation of charity, the love of neighbor, and that part of the Great Commandment, so it's foundational in that sense to his moral theology. But it doesn't explain what the virtue of justice is, except in so far justice is directed towards other human beings.


My interlocutor writes:
But I did read below the Aquinas citation, and was roughly familiar with Aquinas terms for justice and what they mean before. I'm well aware of what you say about not being able to ground modern conceptions of rights and justice in Aquinas. I agree. But it isn't clear to me it does any good to say he "doesn't talk about justice being founded upon the fact that people are equal because they have the same nature," because it seems to me this just begs the question of what relevant thing it is that people share, in part because the statement "people are equal" is pretty vacuous. I'm not interested in defending modern conceptions of rights and justice. Aquinas is famously inexplicit about moral first principles, still what reasons does he give for his supreme one, neighbor-love? What is the rational basis for the golden rule, or is there one?

The New Natural Law theorists have complained that Aquinas doesn't clearly derive the precepts of the Natural Law from human goods and the like. His discussion of the precepts of the Natural Law in I II 94, 2 is admittedly brief. But Aquinas just doesn't have the same project as the NNL theorists, who wish to focus on the precepts in their model of practical reason. Rather, the precepts are associated with the virtues, and subordinate to them. So when he writes,

Hence this is the first precept of law, that "good is to be done and pursued, and evil is to be avoided." All other precepts of the natural law are based upon this: so that whatever the practical reason naturally apprehends as man's good (or evil) belongs to the precepts of the natural law as something to be done or avoided. (I II 94, 2)


We need to know what is good for man but we also have to understand this in the context of his discussion of the virtues. He doesn't reason out the precepts for the reader, but perhaps it should be fairly obvious.
As for charity, charity extends to neighbor on account of God, who desires to share Himself with rational creatures. So those who have the same nature and end, other human beings, are loved with charity. But so are those that do not have the same nature but are higher, the angels.