Fides et Ratio: Aquinas, Greek philosophy, & the proofs for God's existence
In his defining work Summa Theologica, St. Thomas Aquinas articulates five ways to demonstrate beyond a reasonable doubt that there must be a monotheistic, creator God discernible by the use of human reason alone. These ways have famously been known as “proofs” for the existence of God, though they are more accurately understood as ways of arguing for God's existence. The first way is generally known as the way of the “unmoved mover.”
St. Thomas argues it is evident that in the world some things are in motion. Of course, he makes this argument against thinkers like the ancient pre-Socratic philosophers Parmenides and Zeno, who argued that there is no movement. Nonetheless, for the purposes of his demonstration, St. Thomas presumes movement. He then goes on to argue that something cannot put itself into motion; instead, it must be moved by something that is already in motion. Using the analogy of hot and cold, he argues that something cannot be simultaneously hot and cold. Instead, he argues, something is either hot or cold. St. Thomas links this syllogism with the notion of movement and argues that just as something cannot be simultaneously hot and cold, so too, something cannot be both unmoved and in motion. Having established this, St. Thomas argues that he has demonstrated that something in motion must be put into motion by something already in motion and prior to it. He then posits that this cannot go on to infinity; instead, there must be a mover at some point in the chain of causality that was not moved by something prior to it. He states that this unmoved mover is not in motion and yet is the first mover or cause of motion—and that this unmoved mover is God.
St. Thomas' argument from motion differs from St. Anselm's ontological argument for the existence of God because of the approach that the two use to begin their analysis. For St. Anselm, the question about God's existence begins with a theoretical conception: his notion of imagining the most perfect being possible in an interior way and then using reason to arrive at the understanding that there must exist an even more perfect being since a finite being cannot truly conceive an infinite one. St. Thomas, on the other hand, begins his argument from empirical data: one can discern and measure that material objects are moving (pace Parmenides and Zeno). It is also important to note that while St. Anselm's way of understanding who and what God is speaks to God's essence—who God is in his very being and substance, St. Thomas' way is designed to lead the skeptic through human reason to the threshold of revelation. In other words, while St. Anselm presumes that one can imagine a metaphysical reality that cannot be discerned or measured in this world, i.e., infinity, St. Thomas begins with that which is provable by empirical observation.
While St. Thomas' way of the unmoved mover, along with the other four ways, is based upon an observably discernible empirical fact, his proof is not designed to establish beyond all doubt whatsoever that God exists. Indeed, because St. Thomas argues from human reason, even he would admit that these ways are limited. After all, St. Thomas—as a Catholic priest and theologian—certainly believes in the primacy of divine revelation, particularly in light of the Word made flesh, Jesus Christ. This so-called certainty, though, is only a foundation of reason upon which to build a faith-filled response to the truths about God contained in that same divine revelation.
Ultimately, neither St. Thomas nor St. Anselm can prove the existence of God through their ways in any empirical sense. Precisely because God is a supernatural being—that is, “above nature”—he cannot be measured or properly understood using natural reason. Indeed, as the Scriptures say, “For as the heavens are exalted above the earth, so are my ways exalted above your ways, and my thoughts above your thoughts” (Isaiah 55:9 (Douay-Rheims)). This is also clear in Bl. John Paul II's work Fides et Ratio, where he writes, “Since access to the truth enables access to God, it must be denied to none. There are many paths which lead to truth, but since Christian truth has a salvific value, any one of these paths may be taken, as long as it leads to the final goal, that is to the Revelation of Jesus Christ” (Fides et Ratio, n. 38 ¶ 2). Though the Revelation of Jesus Christ—both the written Scriptures and the Word himself—are the goal of Christian teaching and philosophy, this revelation does not abandon reason as a tool through which to understand more about who God is. As a Christian, St. Thomas Aquinas believed that there was only one Truth: “...the Way, the Truth, and the Life...,” Jesus Christ (John 14:6). It is within this encounter between truths—philosophical and theological—that St. Thomas used his ways to reconcile faith and reason for the Truth who was, who is, and who is to come.