The Fifth Way: Aquinas’ Argument from Design

The Argument

  1. Natural bodies act for ends
  2. Whatever lacks intelligence cannot move towards an end, unless it is directed by some being endowed with knowledge and intelligence
  3. Therefore, some intelligent being exists by whom all natural things are directed towards their end
  4. This being we call God

The Defense

Compared to the third and fourth ways, actually understanding what the argument here is trying to do is relatively simple. What is challenging is that the terms the argument uses have, in the past few centuries, accumulated so much baggage that the argument as a whole is very commonly misunderstood and lumped in with an assortment of other, lesser arguments. So we should make this clear from the start: the argument has nothing to do with the debate between evolution and intelligent design. In fact, it really has nothing to do with specifically “living” things at all; it could work even if we removed all living things from consideration.

The first premise is that in the world we observe natural bodies which act for the sake of an end. It is (or, at least, should be) rather undeniable that at least some things in the world act for ends — namely, rational agents. Indeed, acting for ends is part of the whole essence of what it is to be a rational agent. For instance, suppose someone slaps you in the face. Your immediate response (after resisting the urge to hit back, of course) is probably to ask: “Why did you do that?” The question of why is a question about ends. In other words, you are asking: “For what reason/end did you do that?” Human agents act for ends.

What does it mean to act for an end? Saint Thomas gives a helpful explanation in reference to rational agents: “There is no question that intellectual agents act for the sake of an end, because they think ahead of time in their intellects of the things which they achieve through action; and their action stems from such preconception” [1]. Suppose, for instance, you are building a house. Before you do so, you have the idea of the house, you think and deliberate about building the house, and then you decide to build the house. To act for an end means that the action is directed towards or for the sake of something. And we can see that this is true in rational agents because rational agents can think ahead of time of that which they are seeking in the actions they undertake. The end guides and directs the decision to act. 

But Aquinas’ argument begins from the observation that not only do intelligent creatures act for ends, but natural, non-rational things do as well. And this seems, at first, much more suspect. After all, isn’t it rather obvious that natural things don’t think ahead of time about what they’re going to do, or even think at all? Yes, but it still must be the case that they act for ends. Saint Thomas writes: “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” [2].  In other words, we see that things in the universe operate with orderliness and regularity. Fire burns; water freezes in cold temperatures; the moon orbits the earth; animals seek food, water, shelter, and to reproduce within their own kind; and so on.

For Aristotle, these facts were quite a clear and obvious indication that even natural things act for ends. After all, even as you can ask an assailant, “why did you hit me?” you can likewise ask why of some natural phenomenon. “Why do planets orbit stars?” “Why do hearts pump blood?” “Why do lions eat zebras?”

Some might fear that we are risking equivocation by suggesting a similarity in these “whys” and the “why” that is involved in the decision of a rational agent. This is because modern scientific methods have tended to rule out teleological explanations in favor of mechanical explanations, i.e. explanations in terms of efficient causation rather than final causation. For instance, if we ask why fire gives off heat, the answer is going to be a description involving reference to some initial energy breaking chemical bonds in a fuel, which releases thermal energy. Notice that this description is essentially mechanical: the initial energy is the efficient cause of the broken chemical bonds, which are the efficient cause of the release of thermal energy.

But the problem is that efficient causation necessitates final causation. Any efficient cause, qua efficient cause, must act for the sake of some end. After all, as Hume pointed out, it is logically possible that from any action, any effect could follow. There is nothing logically incoherent with the notion of fire being cold to touch rather than hot; it is perfectly conceivable. It is also logically possible that every single time I encounter fire, it produces a different effect. It could be hot one day, cold the next day, loud the next, soft the next, etc.

But this is exactly the opposite of what we observe in nature. As we’ve already said, things in nature operate orderly and with regularity. This regularity is what allows for patterned predictability in scientific experiment, and the creation of theoretical models. It’s what allows for any sort of inductive reasoning. All else being equal, barring any interferences, any and every time I combine two hydrogen atoms with one oxygen atom, I will produce a water molecule. The combination will not result in combustion, or the sound of a violin, or the implosion of a planet billions of lightyears away. Nature is regular and ordered.

To say that an action tends toward a determinate end is just to say that there is a specific effect which the action of that cause is inherently directed towards. If the action of a cause were not determined to a specific effect, it could in principle have any possible effect, of which there are an infinite amount. As Saint Thomas puts it: “From an agent contingently indifferent to alternatives no effect follows, unless he be determined to one effect by something. So, it would be impossible for him to act. Therefore, every agent tends toward some determinate effect, and this is called his end” [3]. 

In short, there are an infinite number of possible effects, so why do causes only result in specific, determinate effects? The only metaphysical explanation is that the causes, by their very nature, are inherently directed to those certain determinate effects as to their ends. If causes did not by nature inherently have specific determinate effects, then there just would be no possible cause-effect relationships at all, and hence no natural regularities, no natural order.

So it is the case that natural things act for ends. This is not to say, of course, that natural things know or are in any sense conscious or aware of the ends for which they act. These things are non-rational; they have no intellect or knowledge.

Which brings us to the next premise: Whatever lacks intelligence cannot move towards an end, unless it is directed by some being endowed with knowledge and intelligence.

Why think this is true? Consider again that in order to act at all, a thing must by its nature be intrinsically directed towards or ordained to its determinate end. A rational agent can move itself towards an end, because it can know the end prior to acting; and hence in a sense the end already “pre-exists” in the mind of the agent. But a non-rational thing cannot know its end, and hence the end cannot “pre-exist” in it in the same way. How, then, can it be directed towards this end? It certainly cannot “move itself” towards the end. It must rather be moved/directed by something else, something which 1) does actually know the ends of all natural things, and 2) has the power to move all natural things to their ends.

There must therefore exist some being which has intelligence and “by whom all natural things are directed to their end” [4]. And this being we call God.


You argued that it is logically possible for a cause to have any of an infinite number of effects. But isn’t a specific cause limited by its nature? It might be logically possible for an eye to hear, but surely it is not metaphysically or physically possible. An eye sees rather than hears because of what it is and how it functions. And isn’t this a purely mechanistic explanation, without need for appeal to final causation?

A mechanistic physical explanation of how an eye sees, and why it sees rather than hears, is certainly possible. But although this explanation might be sufficient to satisfy a modern natural scientist, it is simply not in any sense a complete explanation. An eye sees because it is configured in a certain way, and possesses certain features, which enable it to perceive and interpret light from objects, which just is seeing. This is the physical, mechanistic explanation. And it is, of course, an accurate description of things. It is just not doing the explanatory work which we are looking for here; for it just presupposes the natural, ordered regularity which we are trying to account for. It explains one effect (sight) in terms of more fundamental effects (objects reflecting light, being perceived by eyes, interpreted by the brain, etc.). And these more fundamental effects could be explained by still further effects. But all the while the question still remains of why any particular cause always produces the particular effect it does; and the answer can only be that it is directed to that effect as to an end.

Why think there is only one intelligent being that is directing all natural things to their ends?

For one thing, it is just simpler to suppose that it is a single agent directing all things, rather than multiple. Further, the overall harmonious order of the universe suggest that everything is directed by a single agent, rather than various different ones. Finally, there are several metaphysical proofs which establish this point definitively, which we shall examine in the next question.

Even if we grant that there must be some intelligent being which directs all things to their proper ends, why think that this being is “God”?

Once again, the being in question must 1) have knowledge of all the ends of everything that exists, and 2) have the power to move all things to their ends. Surely this alone is enough to convince that the being could only be that which we understand as God.

But also, consider the following. In human beings, rationality is deeply interconnected with physical brain functions, and dependent upon the overall system of the physical body. Most of the processes of the human body are directed by our brains unconsciously — meaning that, though we overall are rational agents, many of our internal actions are natural, non-rational processes. Which means, as we’ve seen, that they must be directed to their ends by something else that has intelligence.

Now suppose that the being (or beings) which are directing all the ends in the universe are like this, except to a superior degree. Each intelligent “orderer” would order some natural things, but also would depend upon natural processes within itself in order to function, which processes would depend up some further intelligent orderer. Since there cannot be an infinite regress of an essentially ordered causal series (a point we’ve established in previous posts), then there would have to be some ultimate first cause that is pure intelligence, having no non-rational components at all. This being would be knowledge itself, essentially intellect, and hence there could only possibly be one (since, if there were more than one beings which were “knowledge itself,” they would have to differ in some respect that is additional to knowledge, in which case they would not be pure knowledge.) Or, as Aristotle put it, this being would “Thought thinking itself.”

Also consider that to move something towards an end is to actualize its potential, meaning that, as first cause of such movements, this being would have to be purely actual. And we’ve already seen that any being that is purely actual is immaterial, incorporeal, timeless, eternal, and immutable.

And this is what we call God.



[1]. Thomas Aquinas. Summa Contra Gentiles. Book three, chapter 2.6. Accessed here:

[2]. Thomas Aquinas. Summa Theologiae. Translated by Fr. Laurence Shapcote, O.P. The Aquinas Institute for the Study of Sacred Doctrine, 2012. I, Q. 2, Art. 3.

[3]. Aquinas. SCG. 3, 2.8.

[4]. Aquinas. ST. I, Q. 2, Art. 3.

Other sources:

Feser, Edward. Aquinas. London: Oneworld Publications, 2009.

Clarke, W. Norris. The One and the Many: A Contemporary Thomistic Metaphysics. Notre Dame: University of Notre Dame Press, 2001.







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