Thomas Aquinas and the Reasonableness of God

One of the great questions of mankind throughout the ages has been the question of whether the existence of God may be proven with certainty. This question has been especially important to theologians and philosophers. One system of philosophical, theological thought which developed during the Middle Ages that attempted to answer this profound question was known as Scholasticism. Within Scholasticism, Thomas Aquinas stands as one of the most prominent thinkers of this philosophical and theological discipline. Of his most famous works approaching the proof of God, the demonstration of God’s existence in five ways contained in his Summa Theologica stands out in a compelling way. 

In this paper, the person of Thomas Aquinas will be identified, with a brief background about the Scholastic thought which influenced him, followed by a summary of his five ways demonstrating the reasonableness of God’s existence, and will conclude with a critique of Aquinas’ overall argument in light of the question regarding certainty for God’s existence.

To begin, in order to understand the arguments of Thomas Aquinas, a certain familiarity with Scholasticism is necessary. The Scholasticism which developed during the Middle Ages was marked by an effort to have a grand synthesis of all truth. The underlying premise which drove this effort was a recognition that all truth is God’s truth. This meant that wherever one discovered truth, even if it lay outside of the traditional, orthodox teaching of the Church, then it could be assimilated by the Church to help inform theological understanding. This was not necessarily a new concept, being championed by the early Church father, Augustine, but was reintroduced as a rallying cry for those who would later be recognized as leaders within Scholastic disciplines.

Under the premise that all truth is God’s truth, and therefore beneficial for theology, the writings of the patristics, such as Augustine, and the ancient Greek philosophers, such as Plato, were all seen as having a relevant voice alongside Scripture. The Scholastics sought to make a grand synthesis of the truths contained within these sources. One source which rose to prominence, heavily influencing much of Scholastic thought, was the Greek philosopher, Aristotle. For many centuries Aristotle and his writings had been mostly lost. Boethius, a Roman philosopher of the sixth century, had planned to translate the works of Aristotle into Latin so they would remain intellectually viable to Western Europe which was losing its ties to the Greek speaking East. But he fell into trouble with the Emperor of the time and was executed before he could get very far in his translation. As a result, Aristotle slipped further into obscurity until the thirteenth century when Arabian philosophers began translating his works and making them available to the West.

The freshly translated Aristotle was received with differing attitudes among the European colleges which had now become the main platform of higher education. In the University of Paris, his works were initially banned because they were seen as being a potential threat to the Christian faith. But in the University of Naples, Aristotle was embraced and taught openly as a useful tool by which intellectual problems may be addressed and resolved. The attitude of Naples reflects well the attitude of the Scholastic period; where truth was found, it was embraced as God’s truth even if it challenged the traditional views of the Church, and should therefore be assimilated into the synthesis of all truth. Ironically, Thomas Aquinas attended the University of Naples whereby Aristotelian thought became one of his fundamental influences.

As a product of the Scholastic period of time in the Middle Ages, the theologian Thomas Aquinas (1225-1274) stands above the rest. He thoroughly integrated Aristotelian philosophy with Christian theology. Aquinas’ goal “was to create a discipline by which God’s revelation, which is unique and beyond human reason, can be discussed and taught reasonably and systematically – in short, to create a divine science.” 

Driven by this vision, at the pinnacle of his achievements, is one of the great systematic works of all time, the Summa Theologica. Drawing from Aristotle, Aquinas sought the logic of the outside, natural world in order to give rational explanations and proofs for theological principles. With this, he also presented his observations in the Scholastic style of teaching in that he formulated his writing in a dialectical form of disputation. This took on the structure of a question, followed by an objection, followed by a counter objection, then a response, and concluding with any extra replies necessary to address the original question. Furthermore, though heavily influenced by Aristotelian philosophy and Scholastic style, his writing also drew heavily on the theology of Augustine. Armed with these tools, Aquinas gave the world a great work of theology which is still beneficial to the Church today.

One way in which his Summa is still helpful is with the question posed earlier regarding whether the existence of God may be proven with certainty. It has been rightly observed that Aquinas puts forth his theology from the perspective that God’s essence is unable to be grasped by the minds of created beings. As Bauerschmidt has said, “For Thomas…God’s essence is ungraspable not because God hides from us, but because when we turn our minds to God there is too much offered to our understanding.” Therefore, Thomas does not offer much in the way of proving the existence of God from an ontological argument, such as Anselm of Canterbury did with his famous words, “God is that being than which no greater can be conceived.” For Thomas, the transcendence of the essence of God is so far removed from the human intellect that he chooses to spend his philosophical energy on arguments which highlight the effects of God seen through natural revelation.

As Diogenes Allen points out, though God’s effects in the natural world are unable to help mankind arrive at God’s essence because God’s being is greater than the natural world and is not contained by it, this does not mean that the effects of God remain unhelpful in establishing proof of His existence. This too was observed by Aquinas. In his five ways (proofs) for the existence of God, Aquinas works from the effects of God seen through natural revelation back to God as the first cause, thus establishing His reasonable existence.

Aquinas’ approach may be demonstrated in a brief summary of these five ways. The first way, which he calls “the argument from motion,” is a cosmological argument. He argues in dialectic fashion that objects in the natural world have movement in the sense of potentiality to actuality, such as that of an acorn (potential) to an oak tree (actual). These objects which have movement cannot cause their own movement, but instead, must be moved upon by a mover which is in a state of actuality. The object which moves from potential to actual does so through a series of causes. Since it is impossible for there to be an infinite sequence of causes, Aquinas reasons that there must be a first cause producing all of the secondary causes. In conclusion, he states, “Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.”

In his second way, which Aquinas calls his argument “of the efficient cause,” he delivers another cosmological argument from the proof of power. He observes that there is nothing in the natural world that is found to be its own efficient cause for existence since for a thing to be its own efficient cause for existence, it would necessarily have to exist before it had existed. Furthermore, arguing from an perspective of causation, he points out that an infinite series of efficient causes is impossible, reasoning that there has to be a first efficient cause for the natural world. Therefore, he says, “it is necessary to admit a first efficient cause, to which everyone gives the name of God.”

In Aquinas’ third way, he presents his proof of “possibility and necessity.” This is also a cosmological argument pointing to the necessity of a Being in whom it is not possible for it to not exist. He begins his argument by observing that objects in the natural world are able to not exist since they are generated, then corrupted, and finally perish. Therefore, for objects in the natural world, it is possible for them to not exist, which makes it impossible at the same time for these objects to always exist. Furthermore, since it is possible for the natural world to not exist, then it is possible for there to have been a time in which nothing at all existed. If there is a possibility for nothing to have existed, then it is right to reason that nothing should have ever existed. Aquinas points out that this would be absurd since things actually do exist, which draws to the surface the necessary existence of something in which it is not possible to not exist, causing the existence of all other things. In conclusion, he says this something, “all men speak of as God.”

The fourth way for the proof of God’s existence, Aquinas delivers an argument from “the gradation (degrees) to be found in things.” He observes that beings in the natural world possess degrees of qualities. There are some more and some less good, true, noble, and so on. Since these qualities are able to be observed and categorized in degrees, it is right to conclude that there is a highest degree which is the degree of perfection. Aquinas argues that this degree of perfection which is the maximum of a quality is the cause of all other beings which possess the lesser degrees of these certain qualities. The possessor of this degree of perfection being the efficient cause of all other beings possessing these qualities in lesser gradation, he concludes, “we call God.”

The fifth and final way, Aquinas calls an argument “from the governance of the world,” and is a teleological argument pointing to the reality of purpose and design in the natural world. He argues that since things in the natural world which lack intelligence nearly always act in a way which results in a certain end, this action suggests design. But since these things which have design producing a certain end, but do not possess the intelligence to produce that end, there must be a being who exists in order to direct all things toward their end. Again, Aquinas concludes, saying, “this being we call God.”

In a final critique of Aquinas’ five ways for proof of the existence of God, it is noted that he draws heavily from the principle of causation observed in natural theology. For Aquinas, natural theology is the starting point for his arguments, and only after establishing by way of reason the implications of his observations from natural revelation, does he draw from special revelation concluding that God is the obvious answer to the observed phenomena. Some, such as Karl Barth criticized Aquinas for placing too high of a view on natural theology, saying that for mankind to know true truth about God, it has to always begin with special revelation from God. Augustine also championed this emphasis on revelation before reason. 

But yet, it may be argued that God has created mankind in such a way as to be able to reason through observation of the world concluding in proofs which point to the existence of a Designer and Creator. It seems that Aquinas has demonstrated this very well by drawing from his Scholastic influence which recognizes that all truth is God’s truth, including the observable, rational truth of the natural world.

With this critique, it must go beyond the debate of which philosophical approach is best to the question of effectiveness. Does Aquinas’ five ways arguing from natural theology provide a sufficient answer to whether the existence of God may be proven with certainty? The simple, straightforward answer is that his arguments do not provide the certainty of God’s existence, but rather, the reasonableness for God’s existence. Aquinas’ proofs do not carry sufficient evidence in themselves to give to mankind the answer that beyond the possibility of all doubt, God exists. Nor does he put forth his arguments as being able to do so. In all five of his proofs, he states his conclusion in a way which does not assert certainty. But this is not to say that he has not delivered to the world a very strong set of arguments concluding that it is highly probable that God exists, and that it is right and proper to believe He exists rather than that He does not exist. The evidence, by way of reason and observation from the natural world, is overwhelmingly convincing that there is a Designer and Creator who is the First Cause of all things. Therefore, in answer to the original question posed above, no, it is not proven for certain that God exists (at least from the philosophical arguments of Aquinas), but yet, it is reasonable to believe that He exists (as demonstrated by Aquinas). Therefore, in this, mankind may have confidence in the existence of God.

In conclusion, as mankind continues to wrestle with the question of God’s existence, the Lord has given the world a great gift in the Scholastic theologian, Thomas Aquinas. Being influenced by the Aristotelian thought contained within the discipline of Scholasticism, Aquinas has delivered a very powerful set of arguments which are not easily ignored or destroyed, pointing to the confident reasonableness for the existence of God from the observation of natural theology.


Allen, Diogenes, and Springsteen, Eric O. Primary Readings in Philosophy for Understanding Theology. Louisville, KY: Westminster/John Knox Press. 1992.

Aquinas, Thomas. Summa Theologica. Translated by Fathers of the English Dominican Province. London: Burns Oates & Washbourne. 1948.

Bauerschmidt, Frederick Christian. Holy Teaching: Introducing the Summa Theologiae of St. Thomas Aquinas. Grand Rapids, MI: Brazos Press. 2005.

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