John M. Grondelski, here from "Why so much ado about the Ten Commandments?":
Paul is undeterred, arguing that what Israel received in divine revelation, everyman receives in “the law written on men’s hearts” (Rom 2:15). It is a law every man is aware he has at one time violated. He is aware because every man has the experience of obligation (“I ought to do X,” “I ought not to do Y”) yet he also experiences his betrayal of obligation (“I did what I ought not to have done,” “I did not do what I should have”). As these experiences are the lot of everyman (i.e., every man experiences guilt), one can only explain it as man is accountable to a law of which he is not the author. As the young Karol Wojtyła argued in his analysis of the human experience of obligation, man is not the author of that law because, if he was, he could dispense himself. He could waive the duty. But he finds he cannot. The persistence of that sense of guilt indicates that what he violated was more than just his own, self-imposed expectations.
That law “written on the heart” is natural law, the universal human
awareness that “I ought to do good and avoid evil.” But that awareness
is not limited to just that very abstract principle. It does not take a
refined moral genius to conclude that “evil” acts include things like
killing, lying, being unfaithful (or wanting to if I could get away with
it), and stealing (or wanting to, if I could pull it off). The natural
awareness of the status, role, and work of parents leads without too
much mental strain to the conclusion that mothers and fathers should be
honored. You’ve basically got the second tablet of the Ten
Commandments—Commandments IV-X—right there.
