In this paper, I contrast the notion of natural law on a secularist scientific picture with the notion of natural law in the thought of Thomas Aquinas. I show the way in which the highly various metaphysics of the two worldviews give rise to such divergent notions. In this connection, I look at contemporary arguments against reductionism in the sciences and in recent metaphysics. I argue that this new antireductionist approach sits more easily with the Thomistic worldview than with the secularist scientific view.