This paper aims to consider the boundary role of metaphysics in the realm of ethics within the contemporary debate of analytic Thomism in regard to the naturalistic fallacy. Two interpretations of Aquinas's natural law and natural inclinations will be critically analysed. On the one hand, John Finnis's interpretation – New Natural Law Theory –, which excludes the metaphysical realm in the consideration of Aquinas's natural law; on the other hand, Ralph McInerny and Anthony Lisska's approach, which acknowledges the unavoidability of (...) metaphysics in Aquinas’s ethics. Finally, through the analysis of Thomistic natural law made by Dario Composta (1971), the relevance of the first-person experience in identifying what normatively pertain to human nature will be shown. (shrink)