"In referring to natural law, Thomas was talking about the fact that we cannot simply decide the rightness or wrongness of certain actions. Take abortion. You can argue as an ethicist as to whether abortion can be permitted, but you cannot say that abortion is simply a matter of choice, for the simple reason that certain ethical notions are built into the very grammar of our discourse. It's not for us to overthrow them—even though in practice, of course, we tend to blur the categories when it suits us."
David Burrell as interviewed by Rupert Shortt in God's advocates: Christian thinkers in conversation (Grand Rapids, MI: Eerdmans, 2005), 137.