That’s the part on which I needed clarity. I’m unclear as to how human nature provides an objective foundation of right and wrong.
No problem, I’ll try to clarify a bit.
Human nature is social, meaning that for a human to develop they require a social environment with other humans. Social living is not a choice, it is a necessity for the brain to develop. Human nature therefore imposes the necessity of social stability on individual humans.
Individual acts that disrupt the social fabric go directly against human nature; discord doesn’t just cause personal difficulties, it inhibits natural human development. Social order is both an outgrowth of human nature and a requirement for its proper function and development. We can’t be human without socialization.
From the perspective of a human individual our nature is an objective fact, not a subjective affectation or choice; we are objectively social, not merely subjectively so. An adult human who has already developed may choose to withdraw and become a hermit, but this choice can only come because they have already grown through socialization. A hermit may not disrupt the social order (unless they are withdrawing support of those who have a natural right to it, such as a father leaving his wife and children) but certain actions such as rape, theft, and murder directly harm the social fabric and are therefore contrary to human nature itself. These acts are therefore immoral because they are contrary to objective human nature.
Obviously some actions that Catholics consider immoral aren’t immoral under this kind of consideration, but the point isn’t that natural law is identical to Divine Law, but rather that natural law is sufficient to determine some real kind of morality, and therefore the reality of morality doesn’t prove that God exists.
Hope that helps!