While St. Thomas thought his First Way for proving the existence of God was “the most manifest” in his own day, it is the Fifth Way that is the most accessible to modern man. Among the Five Ways, the Argument from Finality speaks most clearly modern man’s anti-metaphysical language. In fact, one modern philosopher, Immanuel Kant, thought the Fifth Way oldest, clearest and the most accordant with the common reason of mankind.” This is a powerful endorsement coming from the man who killed metaphysics and thought that there could be no objective proofs for God’s existence. Given its accessibility therefore, it is instructive for us to examine it more closely.
The Argument from Finality is often mistakenly confused with its doppelganger, the Argument from Design. St. Thomas’ proof is deductive and demonstrative while all the variations of the Intelligent Design Arguments are inductive and probabilistic. The latter always leaves open the possibility, even if it is remote, that there is in fact no Intelligent Designer. The Argument from Finality, while it too comes to the same conclusion, it proceeds in a logically sound manner leaving no doubt as to the existence of a Supreme Intelligence Who created and sustains all things in existence.
The two types of proofs are different in another important way. Like the other Four Ways, St. Thomas’ proof is not really concerned with creation, but preservation. It is concerned primarily with why things are the way they are right now. In other words, it eliminates the possibility of deism that plagues all of the Intelligent Design-type arguments.
The Argument
With that said, let us turn to St. Thomas’ rather brief argument directly.
The fifth way is taken from the governance of the world. We see that things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer. Therefore, some intelligent being exists by whom all natural things are directed to their end; and this being we call God.
ST I, q.2 a.3
Proceeding from direct sense experience of the world, St. Thomas posits that since non-intelligent beings always (or at least when not impeded) act for an end, that is, act intelligently, there must be an intelligence “underwriting” their intelligent activity.
On the one hand this is common sense. In fact, this is such a “given” that empirical science treats it as a first principle. In order for science to proceed, it has to assume that the object of its study is intelligible. Intelligibility requires intelligence. Prediction requires predictability which requires a governing intelligence.
But common sense, especially if it conflicts with a scientistic worldview, is not so common. This makes philosophical inquiry necessary. Framing the discussion within philosophical terms such as final causality makes the argument clearer. Recall that a being can be explained with regard to its four causes. The first two causes, the material cause, or what it is made out of, and the formal cause, or what makes the thing what it is, are intrinsic. The other two, efficient and final causes, are extrinsic. The efficient cause is the external cause that brings about the existence of a thing or a new way of existence. This need not point to a First Cause (at least directly), but can refer to secondary causes. The efficient cause of new oak tree is an acorn. Looked at from the perspective of the acorn, we can say that the final cause of the acorn is to become an oak tree. Given all the right conditions, it will develop into an oak tree and not anything else like a rosebush or a donkey. This is always the case, so much so that we can say that the acorn acts towards this end and not another.
This connection between a thing acting as an efficient cause and fulfilling its own final cause is very important for modern science. For modern science seeks to study efficient causality. In developing predictive models for inert matter, it seeks to explain what causes changes in matter. It does not concern itself so much with final causes, but they are always lurking in the background because of this inherent connection between the two extrinsic causes. Even if it does not so much care about final causes, the modern scientist cannot act as if they don’t exist without simultaneously denying efficient causes. It is like sawing off the branch you are sitting upon.
Why There Must be a Final Cause
Because the acorn lacks intelligence, this inner directedness of the acorn to develop into an oak tree must have an extrinsic intelligent cause. This becomes evident when we realize that Aquinas is talking, not about creation but preservation. Why does the acorn, here and now, have as its end or telos, the oak tree? And why must there be Intelligence for this to be the case? In order for a final cause to be a true cause, then the effect must be in the cause. To see how this works, we will draw an analogy with a human artifact, say a house. The builder is the efficient cause of the house, but he is also what is called the exemplary cause. It is his idea of what the house will look like that is the final cause. That cause does not exist in the house, but in the mind of the builder. So if we return to our acorn, we can ask where the final cause exists. When we do, we realize that it exists as a divine idea. Now we see why the final cause cannot exist without an intelligence. It must first exist as an exemplar in order to be a true final cause. It must exist not just at creation, but also in the here and now.
Once this link between final causes and intelligence is made, we see why St. Thomas’ argument is true. The fact that we observe anything that acts as an efficient cause is also acting upon its own final causality. Because these things act towards ends, and not just any ends, but very specific ends, there must be an intelligence behind it. “Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.”
Understanding what St. Thomas is really arguing for then becomes important because it differentiates it from other Intelligent Design arguments. This demonstrative proof is protected from the “God in gaps” arguments that usually plague these types of arguments. Sharing the same assumptions as modern science, it also makes it especially potent against those who reject God based on scientism.