The writings of Thomas Aquinas span the fields of theology, metaphysics, philosophy, and politics. A particular focus for Aquinas was the interpretation of Aristotle, and he wrote detailed commentaries on many Aristotelian works, seeking both to preserve and to modify Aristotle for the Christian world. Aquinas was guided by the central insight that philosophy and Christianity can and ought to be combined, and this thought runs through the entirety of his corpus.

Aquinas’ Aristotelianism and the Natural Law

The thirteenth century, during which Aquinas lived and wrote, saw the rediscovery and revival of classical Greek philosophy, primarily through the work of the Scholastics (among whom is numbered Aquinas’ teacher Albert Magnus). Perhaps not surprisingly, therefore, Aquinas’ writings attempt to achieve a synthesis between Greek philosophy and Christian theology, despite the obviously pagan character of classical thought. His major contributions to political thought, besides sections of his Summa Theologiae and his On Kingship, take the form of commentaries on Aristotle’s Nicomachean Ethics and Politics.

Essential to Aquinas’ understanding of the relationship between classical philosophy and Christianity is the insistence that the two are not necessarily mutually exclusive. Rather, Aquinas does not adhere to a rigid distinction between reason and revelation as the two competing means by which human beings can acquire knowledge. Instead, he saw philosophy and theology as complementary, with the study of each being an important part of the search for truth. What human beings can learn for themselves through their faculties of observation and reason does not undermine what they can learn by means of divine revelatory law. The significant truths that we can discover through our reason are, collectively, referred to by Aquinas as the natural law. However, the natural law forms only one part of the eternal law, which Aquinas describes as being that which gives order and reason to all things in the universe.

Although the natural law is said to be imprinted upon all human beings, insofar as we possess reason, it is nonetheless the responsibility of each man to discover and follow its precepts through an act of his own free will. In this way, Aquinas retains the concept of free will and individual responsibility, while still prescribing a divine order to the world and all things contained within it. This concept of free will means that, while man-made law can and ought to reflect the teachings contained within the natural law, this is not always the case, and so-called “bad” human law does sometimes exist. In an effort to correct the human tendency to misinterpret or ignore the natural law, God has therefore given man the added gift of divine revelation, which is intended to supplement but not overturn the teachings of the natural law. In particular, divine revelation can help direct men toward those human goods or ends that point beyond the earthly world.

The introduction of the idea that the truest goods of man’s existence may be not merely natural, but indeed supernatural, modifies Aristotle’s view that human happiness is closely related to virtue, the excellent activity of our characteristic human faculties. Aristotle’s presentation does not suggest that human beings might receive any additional guidance in their pursuit of happiness from divine intervention in the form of divine revelation. Rather, for him, it is our reason that is central for meeting our ends. This suggestion and its implications are the central modification made to Aristotelian political science by Aquinas, whose arguments otherwise follow Aristotle in most respects.

Just War Theory

The theory of just war seeks to describe the conditions under which the faithful believer can exercise violence justifiably. The general notion of mutually accepted rules of engagement finds its earliest textual support in the works of Homer and Thucydides, with their repeated judgments regarding the nobility (or lack thereof) of the events that occurred during the great wars of their time. In the Christian tradition, a precursor to just war theory can arguably be found in the Bible’s reference to the justice of violence in which divine intervention plays a part. It is not until Augustine, in the fourth century, however, that just war is explicitly mentioned in writing by a Christian author. In his City of God, Augustine argues that war may justly be waged as long as it is done in the long-term service of peace. Although his argument forbids the launch of preemptive war, it does allow defensive warfare, given that the objective of the fighting is to restore peace. Violence can be justifiably used, however, only by those who possess the proper political authority. This maintains a strict prohibition against civil war, no matter how grave the grievance.

It is upon the brief but important remarks of Augustine that Aquinas founds his own theory of just war, a theory that has become the traditional model from which most further discussions of the topic begin. Aquinas outlines the specific conditions that must be met in order for a war to be considered just. First, the war must be undertaken by the state, or the proper political authority. Outlawing any civil rebellion, Aquinas writes that “…it does not belong to a private individual to make war, because, in order to obtain justice, he can have recourse to the judgment of his superior [his prince or similar political ruler]” (Summa Theologiae II-II, Q.40, Art. 1). Second, the war must have a just cause; “those attacked must have…deserved to be attacked” (Summa Theologiae II-II, Q.40, Art. 1). Finally “it is necessary that the intention of those who fight should be right; that is to say, that they propose to themselves a good to be effected or an evil to be avoided” (Summa Theologiae II-II, Q.40, Art. 1). Aquinas devotes relatively little space to expounding the general principles of just war. The task of expanding Aquinas’ words fell to his theological-intellectual inheritors. Most notably, Francisco de Vitoria and the associated School of Salamanca offers a more detailed explication of just war, arguing, for example, that killing innocents or hostages is unjust. At the same time, Vitoria allows preemptive war, so long as an enemy attack is seen as imminent.

Theology and the Existence of God

Aquinas insists that the existence of God is self-evident, insofar as “…things are said to be self-evident to us the knowledge of which is naturally implanted in us” (Summa Theologiae I, Q.2, Art. 1). This is not to say, however, that knowledge of the existence of God is known to man without the use of his reason. Man gains proof of God’s existence only when he devotes his mind to the task of discovering it. As to what he will inevitably discover if he devotes himself to the task of considering the existence of God, Aquinas presents five proofs. First, Aquinas posits the Aristotelian notion of a First Mover, which itself is unmoved by anything else (thus rejecting the notion that motion results from an infinite causal chain), and adds that any such First Mover is universally understood to be God. Second, because causality itself cannot exist in an infinite chain, there must be a First Cause, also called God. Third, insofar as we attribute particular qualities, such as goodness and truthfulness, to individual objects, there must also exist some being that represents the superlative form of all these qualities, a being that is the best and the truest. That being, Aquinas argues, is God. In the fourth place, Aquinas observes an innate tendency of all things to tend toward a specific end or goal, even those objects that seem to lack any self-awareness. Therefore, one must assume that those that lack self-awareness must be receiving guidance concerning their natural ends from a supremely aware being. Thus all knowledge of ends and goods must come originally from God. Finally, Aquinas develops an argument from necessary existence. Of all the beings presently in existence in the world, Aquinas claims, none of them exist necessarily: they are contingent; it is possible for them to not exist. Therefore, because all the things that exist could once not have existed, there must be some other being (God) that exists necessarily and permanently, making it possible to bring all contingent objects back into existence from their previous temporary state of non-existence.

Although these five proofs are not meant to prove the existence of God definitively, Aquinas intends them to be the foundation for all further study.

For further introductory reading, see:

John Finnis, Aquinas: Moral, Political, and Legal Theory, New York: 1998.