My Reconstruction of the Fifth Way

Here is how I would reconstruct the essential argument of the Fifth Way:

1. Some non-intelligent agents have ends.
2. Every agent that sets its own end is intelligent.
3. Any agent that has an end and that does not set its own end has its end set by something else that also has ends. (Seriality)
4. The end-setting relation is transitive, asymmetric, and well-founded.
5. Therefore, every non-intelligent agent that has an end has its end set by some intelligent agent. (2-4)
6. Therefore, there is some intelligent agent. (1, 5)
7. Any contingent intelligent agent has its end set by something else that also has ends.
8. Therefore, there is a necessary intelligent agent. (4, 6, 7)

The premise 1 and the sub-conclusion 5 are explicitly stated in the Fifth Way (Summa Theologiae I, Q2, a3). I get premises 2 and 3 from a parallel passage in Thomas’s Summa Contra Gentiles I.44, where Thomas is proving that God is intelligent:

[7] Again, that which tends determinately to some end either has set (praestituit) itself that end or the end has been set for it by another. Otherwise, it would tend no more to this end than to that. Now, natural things tend to determinate ends. They do not fulfill their natural needs by chance, since they would not do so always or for the most part, but rarely, which is the domain of chance. Since, then, things do not set for themselves an end, because they have no notion of what an end is [they do not cognize the concept of an end], the end must be set for them by another, who is the author [institutor] of nature. He it is who gives being to all things and is through Himself the necessary being. We call Him God, as is clear from what we have said. But God could not set an end for nature unless He had understanding. God is, therefore, intelligent.

Thomas doesn’t state premise 4 explicitly, but this sort of premise has been stated more or less explicitly in the first three Ways, so Thomas does not feel the need to belabor the point here. The setting of something’s end is a kind of causal relation, so it is plausibly asymmetric, transitive, and well-founded. That is, there should be no circular causation and no infinite causal regresses. Premises 1 through 4 validly entail Thomas’s explicit conclusions 5 and 6: there must be intelligent agents at the root of the end-seeking behavior of all non-intelligent agents.

Of course, Thomas believes that all non-intelligent agents act for ends. Everything that acts acts in a determinate way, and anything that acts in a determinate way “intends an end,” in Thomas’s technical, Aristotelian sense.

If the Fifth Way is to be relevant to the existence of God, we need somehow to relate the conclusion 6 to the necessary first cause whose existence has been established in the first three Ways. To get there, all we need to add is premise 7: that any contingent agent (whether intelligent or not) has its end set by some other agent. And that other agent, like all agents, must itself act for one or more ends. This is a causal principle close to those employed in Ways two and three. As a result, we obtain the desired result: there is a necessarily existing intelligent agent.

At this point, we haven’t yet established the uniqueness of the intelligent First Cause. We haven’t yet ruled out that all contingent action could be the result of a plurality of necessarily existing intelligent agents, each of whom sets its own ends.