St. Thomas Aquinas maintains that there are four types of law: eternal, divine, natural, and human. The distinction between Natural Law and human law is where conflict arises and the question of civil disobedience becomes
relevant. Eternal law is the unchanging moral law that St. Thomas Aquinas upholds as deriving from the Divine Source, God.

St.Thomas ascertained that “the received is in the receiver according to the mode of the receiver” (On Human Nature 136). That is, a being can only receive knowledge in a way and form that is suited to their own form of being. Therefore, humans cannot know God’s law as God does, given the distinction between human beings and God. He also posits,“There are two ways in which a thing can be known,” either “in itself” or “in its effect” (Summa Theologiae I-II Q. 93 A. 2). Humans know Eternal law in this second way. He states, “No one except the blessed in heaven . . . can know the eternal law as it is in itself. However, every rational creature knows the Eternal law with respect to more or less what radiates from it” (ST I-II Q. 93 A. 2). By this, Aquinas means that humans cannot know Eternal Law in its essential form, as does God, but only through its derivatives and effects. This phenomenon is defined as Natural Law, which is the rational human participation in Eternal law; it is the law we arrive at through reason, and that to which we adhere on moral grounds, both universal and eternal.

While humans cannot know eternal law, Natural Law, according St. Thomas Aquinas, is our rational
participation in Eternal Law. How are we to know Natural Law? Aquinas asserts, “The first precept of law is that good ought to be done and pursued and that evil ought to be avoided”(ST I-II Q. 94 A. 2). This provides the foundation for all other precepts, which are the “practical precepts” or the general rules by which we might govern our practical actions.

Conflict appears when Natural Law and human law overlap, as they will interact in one of two ways. In an ideal scenario the two will comply: the human law will affirm what is morally just and expressed in the natural law. Otherwise, the two can fail to comply. St. Thomas maintains that natural law is superior to human law as it is humankind’s rational participation in God’s law, which is the highest and most absolute authority—it is difficult to imagine the rationale explaining how the arbitrary rule of humans could surpass the moral obligations we have
derived from God’s law. Given this context, we can understand Aquinas’ statement: “If in any point [human law] deflects from the law of nature, it is no longer a law but a perversion of law” (ST I-II Q. 95 A. 2). Aquinas does not intend to say that an unjust human law ceases to exist. Rather, he believes its authority is nullified. He writes, “Laws can be unjust . . . by being contrary to the divine good, as are tyrannical laws that induce men to idolatry or to do anything else that is contrary to divine law. It is not permissible to obey such laws in any
way at all” (ST I-II Q. 96 A. 4). Natural Law, as the human expression of Eternal Law, carries an analogous level of authority. One can logically infer that an unjust human law—one that violates Natural Law—is not only a law that it is morally permissible to ignore, but is a law that one is morally obligated to disobey.