Aquinas’ natural law theory contains a set of basic goods, such as survival, reproduction and the pursuit of truth. However, whether and how there is a hierarchical relationship among these goods remains disputed. Given the importance of Aquinas’ natural law theory for Christianity and the philosophy of law, this issue merits a closer investigation. By carefully examining various modern scholars’ theories and Aquinas’ texts, it is demonstrated that according to Aquinas, firstly, there are hierarchies of basic goods and sins; secondly, (...) these hierarchies are horizontal and vertical according to the order of participation and the dignity of substances, respectively.Contribution: This research reconsiders the modern debate over Aquinas’ hierarchical theory of basic goods and provides a more authentic understanding of Aquinas’ own view, which can be applied to his theory of sin. Aquinas’ natural law theory can hence be clarified in a more profound way. (shrink)