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Here are some of the difficulties in making sense of the Fifth Way:
 
1. The gap problem seems especially severe. The only conclusion that seems to follow is that every unintelligent natural substance has an intelligent cause. This could be a large number of finite intelligences.

2. No good reason is given for the crucial inference from natural teleology to intelligence. What’s the middle term supposed to be? Given the relatively deflationary understanding of final causation that Aristotle and Aquinas share, there doesn’t seem to be any definitional connection between something’s acting for an end and its being directed by an intelligence to that end.

3. What’s the relationship supposed to be between the Fifth Way and the other four? Are we supposed to be relying on the conclusions of the other ways? If so, how? Is it intended to add intelligence only to the characteristics of a first cause, or is it also supposed to provide some reason to suppose that there is just one first cause?
 
Starting with 3, my guess is that the Fifth Way is supposed to provide us with information about the first cause (or causes) whose existence has been established in the first four ways. This is suggested by the Reply to the second objection in this article (STh I.q2.a3). If so, this helps with the gap problem. The Fifth Way is not intended to stand on its own but to extend the arguments of the preceding Ways.
 
In SCG I.44, in which Aquinas argues for God’s intelligence, there is a passage that closely parallels the fifth way (paragraph 7). In paragraphs 4 and 6 of that article, Thomas infers God’s intelligence from the intelligence of some of His creatures. In particular, in paragraph 6, Thomas appeals to the same thesis that is the conclusion of the Fourth Way, namely, that God possesses all perfections possessed by any existing thing. He asserts that intelligence is a perfection, and consequently God must be intelligent, given the existence of intelligent creatures. We could think of the Fifth Way as simply plugging a loophole. Suppose that a critic objects to the claim that intelligence is truly a perfection, as opposed to a mere by-product of the interaction of natural bodies). The Fifth Way could provide independent support for the intelligence of any First Cause.
 
Alternatively, we could interpret the Fifth Way as an a fortiori argument: if even unintelligent finite substances need an intelligent cause, how much more so must the cause of an intelligent finite substance be intelligent? The Reply to the second objection does allude to the fact that human intelligences need an intelligent cause, since they are changeable, finite, and contingent.
 
Let’s turn to the middle term problem (problem #2 above). The parallel passage in SCG I.44, paragraph 7 provides the key. “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.” This is a kind of PSR or principle of causality, and it doesn’t mention intelligence at all (whether absent in the effect or present in the cause). Applying this principle in the way modeled by the first four ways yields the conclusion that there must be some agent that sets its own end. This interpretation has the advantage of bringing the Fifth Way in line with the first three Ways.
 
How is it possible for a substance to set its own end? Prima facie this is impossible, since anything a substance does (including ‘setting’ or ‘preastituting’ something) is explainable as directed to that substance’s end. The only solution to this problem would involve something’s having the Good itself as its end, ‘setting’ itself toward that end simply by grasping the good as good. Only this could eliminate the element of arbitrariness in the constitution of the First Cause’s end. Moreover, this good that is grasped must be identical to the substance itself, or else it would be being moved by something else and so wouldn’t be a first cause in the order of final causation. Thus, a First Cause must be Goodness itself, grasping itself as good in an act of self-understanding.
 
The Fifth Way, therefore, explains why the First Cause couldn’t be an unintelligent thing, possessing some end “by nature.” Why couldn’t a First Cause be its own end, acting for the sake of itself naturally but unintelligently? How could such an unintelligent first cause cause something else? It would have to be naturally directed toward causing such an effect, and the fact that it is so directed would be arbitrary and beg for a further explanation. Such unintelligent direction toward an end would be a brute fact and therefore incompatible with the First Cause’s being necessary in and through itself. There would be an element of contingency or in-principle explainability about the First Cause’s so acting. Such a First Cause wouldn’t be in principle uncausable, since we could imagine something else causing it to be directed toward creating the actual creature.
 
In contrast, if the First Cause is intelligent, then it can create things simply because they are good (by way of participating in God’s goodness). No unexplained contingency enters into the process, except for the contingency of rational free choice. The contingency of rational free choice is compatible with a strong principle of causality or PSR, since each rational choice can be adequately explained in terms of necessary facts (i.e., the intrinsic value of the action taken).
 
We can appeal to a principle stated in Thomas’s Commentary on Physics II.8. “But because they [unintelligent creatures] always act in the same way it is clear that they do not act by intellect but by nature. The artisan judges the form of the thing built and can vary it.” Both intelligent and unintelligent agents are determined to act as they do by their ends, but unintelligent agents always purse these ends in the same way, while intelligent agents can purse the same end in different ways, since an intelligent agent can grasp intellectually which actions are and which are not effective means for the end, and the intelligent agent can choose any of the many effective means (if there are many such).
 
A first cause that causes other things to exist (i.e., a first cause that acts as a creator) must be intelligent, since it must act in more than one way in pursuit of its end. Every such first cause effectively pursues its end simply be existing in a state of perfection. To create is to pursue that end in at least one additional way. To create multiple creatures because each of them is good in some way is to exercise intelligent choice in still further way.
 
One might question whether it is true that each unintelligent agent must pursue its end in a single way. Why couldn’t it have many ways of doing so, with the actual way or ways being selected not by intelligent choice but by random chance? If that were to be the case, then there would have to be some objective probability of the first cause’s causing any particular creature. We could then ask why the objective probability has the value it does. This would require some further cause to “set” this objective probability to its actual value. Only a rational and perfectly free agent can be a genuinely uncaused agent, since it can choose its own actions in light of its knowledge of the value of the chosen actions, where both the knowledge and the value are necessary in themselves.