Answer: The Five Ways of Thomas Aquinas are the primary rational arguments used by Aquinas to defend the existence of the Christian God. While the Five Ways are commonly mentioned in discussions of history and philosophy, they are easily misunderstood. Critics have alternatively over-complicated, over-simplified, or simply misinterpreted what Aquinas intended with these statements. His true intent was to demonstrate a general, objective, rational case for God’s existence using commonly available observations.

One common mistake is to assume that Aquinas intended the Five Ways to be a complete, airtight case for the existence of God. In reality, he considered them to be only the beginning, a way to support the existence of God to those who cared only for arguments grounded in reason and observation. As such, the Five Ways are better viewed as an introduction to the idea of God’s existence, not the entire sum of Christian theology.

On the other hand, some critics of the Five Ways over-simplify them. This is often combined with misinterpretation. Aquinas’s work was completed in the 13th century, and so the terminology he uses is subtly different from modern vernacular. Aquinas’s use of the word motion, for instance, was meant in the sense of “change,” not physical travel. Interpreting the Five Ways requires careful consideration of Aquinas’s actual intent when he laid out the arguments. Taking the statements in an overly simplistic way or without an understanding of Aquinas’s other philosophical statements is an unfair and misleading approach.

There are many different styles of presenting Aquinas’s Five Ways. Their (relative) simplicity can be deceiving; any one of these five assertions can be dissected, nuanced, and debated endlessly. For the sake of discussion, the primary claims can be summarized as follows:

I. The Argument from Change (“Motion”)

Change is immediately apparent in the universe, in the sense that things move from a “potential” state to an “actual” state. But this potential is for something that does not yet exist and so requires something else to actualize it. Whatever actualizes that, in turn, would have to be actualized by something else. Logically, this chain of changes cannot be infinitely long, or else nothing would have ever changed in the first place. Therefore, there must exist some un-changed and un-changing thing that actualizes all other changes. This principle is not related to time or a sequence of events. Rather, it points out the need to have something capable of causing the changes we observe: God, the Un-Moved Mover.

In other words, the first of Aquinas’s arguments for God’s existence points out that all changes are the result of some other change. But this chain of changes cannot be infinite, so there must be some un-changed (un-moving) thing (an un-moved Mover) that is ultimately responsible for all other changes (motion).

II. The Argument from Causality

Cause and effect are apparent in the universe. Everything that occurs is caused by something else. All events are dependent on some other occurrence or thing in order to make them happen. A thing cannot be the cause of itself, or else it would never come to exist. Logically, this chain of causation cannot be infinitely long, or nothing would ever have come to exist in the first place. Therefore, there must be an un-caused thing that causes all other things. This argument is not related to time or a sequence of events. Rather, it considers the fact that all things are dependent on something else for their existence.

In other words, the second of Aquinas’s ways to show God’s existence is based on the fact that all effects are caused by some other event, which in turn is the effect of some other cause. But this chain of causality cannot be infinitely long, so there must be some un-caused cause: God, the First Cause.

III. The Argument from Contingency

Nothing we observe in the universe is necessary; nothing needs to exist, in and of itself. We often observe things that cease to exist, falling victim to death, destruction, or decay. Eventually, all non-necessary things cease to be. But, if it were possible for everything to cease to exist, and if there has been an infinite amount of past time, then all things would have already ceased to exist. There would be nothing left at all. The fact that anything exists at all, even now, means there must be one thing that cannot cease to exist, one thing that must necessarily exist. There must be one thing that is non-contingent—i.e., its existence is not dependent on anything else. This thing must be.

In other words, Aquinas’s third argument or way to prove God’s existence is that, if everything were impermanent, eventually everything would cease to be. Therefore, there must be at least one thing that must, necessarily, exist (one non-contingent thing): God, the Necessary Being.

IV. The Argument from Perfection

Every trait we see, in every object, is compared to some standard: health, morality, strength, and so forth. The fact that we instinctively see degrees in these areas implies that there is some ultimate standard against which to judge that property. And all comparative properties share a common sense of “perfection.” This means there must be some ultimate standard of “perfection” from which to judge all other properties; those objects cannot be the source or definition of that property in and of themselves.

In other words, Aquinas’s fourth argument in favor of God’s existence points out that, in order to speak of “goodness” or “power,” we must have an absolute standard against which to judge those terms; there must be some other thing from which they ultimately derive that characteristic: God, the Ultimate Standard.

V. The Argument from Purpose

Many things in the universe “drive” toward a particular end, not random results. Magnets “drive” to seek metal or to align their poles. Seeds “drive” to become adult plants, not animals. This regularity, as opposed to randomness, is a sign of purpose—of intention or intelligence. However, magnets and seeds and such have no intelligence of their own. Therefore, their “drive” must be the result of some external intelligence setting or fixing or designing their behavior. In some means or mechanism, all purposes and functions must originate in some intelligent entity.

In other words, Aquinas’s fifth way to show the existence of God involves the fact that inanimate matter and energy do not exhibit intelligence or purpose. When we see something unintelligent that appears to have some specific purpose or that fulfills some purposeful role, we must assume that thing to have been given that purpose by some other intelligence. Ultimately, this leads to God, the Grand Designer.

The Five Ways Today

As we can see, there are strong similarities between Aquinas’s Five Ways and many other common arguments for the existence of God. However, there are distinctions to keep in mind.

Aquinas’s first three arguments share a common theme: that causality, logic, and so forth lead to an inference of the existence of some deity. The most frequently used of these in the modern world is closely related to the second argument, causality, and is usually simply referred to as the cosmological argument.

Fundamentally, the fourth argument is almost identical to the ontological argument as presented by Anselm. Aquinas saw a distinction, however, as his focus was on the source of a thing’s existence. That is, Aquinas was arguing that goodness or power in some finite object can only come from some other, greater source. Anselm’s ontological argument, technically, is more focused on the generic concept of “perfection.” Still, it is not uncommon for Aquinas’ fourth way to be approximated as the ontological argument.

The fifth argument, also known as Aquinas’s teleological argument, is similar to the modern-day argument from Intelligent Design. However, Aquinas’s argument presumes that individual components have some form of drive or initiative in and of themselves. Intelligent Design, on the other hand, presumes that individual components (e.g., atoms or energy) have no particular purpose or function outside of an intelligent intervention. This distinction is trivial for most purposes today. Yet, strictly speaking, Aquinas’s fifth way is not the same as modern Intelligent Design.

Scholars continue to debate the validity of the Five Ways of Thomas Aquinas. Regardless of how useful one considers them in a modern context, their importance in the fields of theology and philosophy cannot be overstated. When properly understood as the “ground level” of a rational defense of God’s existence, they are useful tools.