For Aquinas, everything is regulated by eternal law or God’s creation order (I-II, Q.91, a.3), so it is not surprise that he understands natural law as participation of the eternal law, where natural law is of a general and teleological character (See I-II, Q.91, a.2). In Aquinas’s vision, natural law is linked with divine reason rather than in the will of God as theologians such as Calvin would suggest. This opens the door for considering Aquinas’s notion of natural law to be expansive in the sense that natural law is associated with human’s capacity to reason and producing reflection. When Aquinas affirms natural law participates in the eternal law, he means that the humans as rational beings have the ability to act virtuously in accordance with divine order which includes God’s plan for the universe and humankind. By participating in divine reason, therefore, human actions can fulfill their due purpose by promoting the common good and thus avoiding what is evil. Through this way humans can know more about God and his proposes.
In Q.94, a.2. Aquinas argues that the precepts of natural law are of practical reason. For him, good has to be done and evil must be avoided as mentioned earlier. Natural law, therefore, has a moral dimension: It dictates what is right and must be done vs. what is wrong and must be avoided. Having this in mind, the precepts of natural law must be in agreement with the good and natural inclinations of humankind towards the common good. In this respect, the first natural inclination is self-preservation. This aspect is common of all things, that is, all nature. The second inclination has to do with what is common with animals: procreation and raising offspring. This deals with the living being’s inclination to procreate and multiply, and aspects regarding the relation between male and female. The third inclination has to do with those things that are related with other rational creatures such as discerning what is truth from what is not, living and forming societies, and knowing God.
Noteworthy is that for Aquinas natural law must not be equaled with these natural inclinations. Natural law in Aquinas’s understanding is reasoning which leads people to act rationally and virtuously because reasoning is proper to human nature. In this regard, natural law is objective and universal. In this respect, Aquinas understands natural law as the same for all people (from a theoretical reason), although their particular applications might be different among the people (from practical reason). Therefore, natural law itself in its primary precepts never changes, except what it is derived from it. These derived precepts are particulars conclusions (not self-evident) which agree with the primary precepts but can change depending on the context. Thus, these primary precepts of natural law are teleological in principle and show humanity general goals to purse or follow while the secondary or derived precepts would be the applications of these general goals which emerge from exercising reasoning.
Unlike natural law, divine law comes out of divine revelation. For Aquinas divine law is formed by the Old Law and the New Law. This two-fold division, in general terms, corresponds to the two parts of the Bible – Old and New testament (old law vs. new law). Divine law informs human reason because humanity cannot grasp divine will in its totality, and this makes necessary God reveals those things which are incomprehensible to human reason.
Regarding the purposes of divine law, in Q91, a.4, Aquinas first states that divine law leads and guides humanity in the fulfillment of their end goals. Second, due to uncertainty of human judgments, there is a diversity in people’s judgment about human actions. Divine law clarifies what human acts are good and what are evil. Third, clarity about interior human acts is also needed because human laws cannot discern the rectitude of the human heart. The fourth purpose of divine law has to do with the prohibition of all sins and evil, something human laws cannot regulate them at the same time.
As one observes divine law and natural law are in accordance but the first one goes beyond in scope since it also includes positive law. Divine law brings a better discernment about the heart of the human person, in contrast with natural law which mainly regulates the relationships with others. In this regard, divine law – via divine revelation – reveals humanity the ultimate purpose of human existence which is God himself through the Ten Commandments and Jesus’ teachings, while natural law confirms such revelation and serves as the basis of human laws.