“The existence of God can be shown in five ways.” So begins “The Five Ways” by Thomas Aquinas. It is impressive that such a feat can be accomplished in just five paragraphs. It should be mentioned that these Five Ways are philosophical in nature, and do not dive into too much detail. However, what is here is masterful and should be known not only by philosophers, but by every believer in God.
The First Way: Motion
Aquinas begins with the idea of motion. This is something observable by everyone, and any middle school science class will tell you that “everything which is in motion is moved by something else.” However, we run into a problem. If that which caused the motion is itself in motion, something must have caused it to be in motion also, and whatever caused that motion, if it be in motion, must have a cause for its motion, and on, and on, and on. Aquinas concludes that “we cannot proceed to infinity in this way, because in that case there would be no first mover, and in consequence, neither would there be any other mover; for secondary movers do not cause movement except they be moved by a first mover.” We find we must come to a stop at a first mover which is moved by nothing: what we understand God to be.
The Second Way: Causation
The second Way is like the first. Instead of a first mover, Aquinas points out that nothing can be the cause of itself. For something to cause itself, it must be before itself, already in existence. This is an impossibility. Also, we cannot think that there are an infinite number of efficient causes, otherwise we would never get to the first efficient cause. Without the cause, there is no effect. So, because we are witnesses to effects every day, and there must be efficient causes to them, there must be some first efficient cause, which we call God.
The Third Way: Contingency
Aquinas turns his attention to “contingency” and “necessity.” Simply put, there are things in the universe capable of existing and not existing, of being and not being. However, not everything in existence can be of this kind, “because anything which is capable of not existing, at some time or other does not exist. If therefore all things are capable of not existing, there was a time when nothing existed in the Universe.” The logical conclusion is if there was a time when nothing existed in the universe, nothing would exist now, because as we have seen in the second Way, nothing can bring itself into existence. Aquinas concludes by saying that not all things that exist are contingent, and there must be something that exists that is necessary, “not having the cause of its necessity from any outside source, but which is the cause of necessity in others.” This “something,” Aquinas says, is what we call God.
The Fourth Way: Degrees of Excellence
The fourth Way draws attention to things we might describe as “good,” “noble,” or “true.” At any time, something may be “more” or “less” good or noble or true than something else, meaning we are using a standard of highest degree to describe certain things. If something is better than something else, it exemplifies goodness in a higher degree. Aquinas’ point here is that there must be something that exemplifies the highest degree, the same way a fire that is hot in the hottest degree is the cause of all hot things (referencing Aristotle’s Metaphysics). So, something must exist which is the cause of being, because it exemplifies being in the highest degree, and this we call God.
The Fifth Way: Harmony
This last way is what we often call the Teleological Argument. Aquinas shows that there are things that lack knowledge (natural bodies) that seem to be working to an End. He notices that they “operate in the same way so as to attain the best possible result.” Much of what we witness in nature does not seem to be mere chance, but has the indication of purpose. But, something lacking knowledge cannot work toward a goal unless it is guided by something with proper knowledge (think of an archer with an arrow). Aquinas’ conclusion is “there does exist something which possesses intelligence by which all natural things are directed to their goal; and this we call God.”
There is much more that has been said and written on the topics discussed by Aquinas, but I believe he has delivered something to believers and non-believers alike that can be a foundation for debate within the realm of philosophy and religion, despite how you may differ.
 Thomas Aquinas, Summa Theologica, trans. Laurence Shapcote (London: O. P. Benziger Brothers, 1911).