Thomas Aquinas' Summa Theologiae

In Thomas Aquinas' Summa Theologiae, he recognizes four types of law. Those four types of laws include Eternal law, Divine law, Natural law, and Human law. If they were to be arranged in a hierarchy, eternal wound be at the top, then divine, natural, and human. Thomas Aquinas' natural law theory is connected to his conception of the "great chain of being". This is the moral ranking of beings, the natural order of society. God is placed at the top, followed by humans; which are ranked from kings to lords to serfs, followed by non-human animals, and plants.

This was a traditional view and structure of the world. Eternal law refers to the laws that govern the nature of the universe. Eternal law states that the whole universe is governed by a divine reason and that everything is governed by God, the ruler of the universe. This is identified as not being distinct from the mind of God, but the reflection of God in his own mind. God's supreme reason is unchangeable, and thus eternal. Since God is the supreme ruler than it stands to reason that he is the supreme originator of law.

Divine law is derived from Eternal Law and is rendered knowable to human thought revelation. As divine law was originated from eternal law, it was circulated to people through pieces of writing or word of mouth. According to Aquinas, divine law is necessary for leading one towards fulfilling their end. Divine law is split between New law and Old law, as they correspond to the testaments of the bible. Aquinas believes that it is necessary to have divine law because as he mentions, "it is by law that man is directed how to perform his proper acts in view of his last end.". Human judgement is incomplete and cannot reason law, justice, and morality just from nature and society. For Aquinas, natural law is based on reason. Natural law can be understood by reflecting on the nature of God's purpose in creating man. Since all things are made by God, they possess purpose since God does not make purposeless things. As a human species, we must determine what is best for us by finding our natural reason, in order to achieve proper acts and end. Internally, we possess our own natural function, which is the reason which allows us to figure out the natural.

Natural law is understood when we humans participate to achieve our proper end through reason and free will. Aquinas thinks of human laws as rules made for governing human society. Laws are only laws if they reflect the natural and eternal law. According to Aquinas, if they do not reflect natural and eternal law they are not laws because they are not 'lawful', as he states "a law is nothing else but a dictate of practical reason emanating from the ruler who governs a perfect community." According to Aquinas, human law is subject to everyone, as everyone has to obey the law. The ruler of a society is obligated to follow the law that they have stated, but there is no one person over them to judge. Experience in practical matters allows us to improve laws, therefore they are subject to change.