St. Thomas Aquinas' Five Ways to the Existence of God

            This Advent we’ve made a point to talk a lot about who God is and what kind of life He is calling us to. We’ve made the point that we have a choice to follow God’s will or our own will, to obtain meaning from God or to live without meaning and purpose, because God made everything with meaning and purpose, but without God everything is just the result of a series of accidents. So, how can we be sure that God really exists? Do we just have to take the best guess with no real evidence? 
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            There are two ways to know something: in itself or from its effects. For example, you can know a person because you met them or because you’ve seen the things that they did. St. Thomas Aquinas believes that we cannot know God in Himself, because He is mysterious and out of our reach (He does reveal Himself to us, but this is Revelation, not reason). Therefore, St. Thomas Aquinas argues for the existence of God from His effects, that is, from the world around us.

            First, we have to have a starting point, so we’ll start with something that we can’t prove, but that is obvious and irrefutable. Nothing comes from nothing. To put it another way, you can’t get something from nothing. Everything has a sufficient reason for its existence. This is called, fitting, The Principle of Sufficient Reason. From this assumption, St. Thomas Aquinas has five ways, or arguments, to come to the existence of God. I’ll summarize two of his arguments today, and we’ll come back to the other three another time. Notice that these ways don’t just try to prove that God exists; they also try to tell us something about God. The original text of the Five Ways can be found in St. Thomas Aquinas’ Summa Theologiae, in the third article of the second question of the “Prima Pars.”

            The Argument from Motion begins with the fact that some things in this world are in motion. We know that whatever is moved is moved by something else. For example, in pool the balls move because they are struck by the cue ball. However, the thing that moves the first thing must also be put in motion by something else, just as the cue ball is first put in motion by the pool cue. This cannot go on infinitely, because then there would be no First Mover and therefore no motion. Therefore, it is necessary to arrive at a first mover, put in motion by no other, and this everyone understands to be God.

            The Argument from Causation is similar to the Argument from Motion. Based on the Principle of Sufficient Reason, we know that nothing can cause itself to exist. An oak tree, for example, comes from an acorn, but the acorn must also be planted in good soul and get enough water for an oak tree to grow. These things are all causes of the oak tree. Now, there’s an order to causes. The acorn that grew into the oak tree came from another oak tree, which came from another acorn, and so on. This procession of causes can’t keep going forever. If there is no First Cause, then there can be no subsequent causes, and then nothing would exist. If there was no original acorn or oak tree, then none of the ones that came from it could exist. However, we know that things do exist. Therefore, there must be a First Cause, and this everyone calls God.

            This Friday we’ll celebrate Christmas and the fact that this First Mover and First Cause entered the world as a little baby born to the Blessed Mother. He isn’t only the distant God of the philosophers; He is also fully revealed to us in the person of Our Lord Jesus Christ.