A basic principle of Aquinas' philosophy of law postulates that all positive laws, the rational human laws, are derived from natural law. The natural just is the normative principle that makes obligatory to human laws. This form of derivation of positive law from natural law is known as determinatio. We wanted to address this issue with the next paper. For this, we focus on two of the passages of the work of Aquinas: the q. 95 a. 2 of the I-II and the comments to the Vth book of Aristotle's Ethics.