The Second Way

​Aquinas's second way, the way of efficient causation, had perhaps the most influence on subsequent natural theology. It has counterparts in the work of Scotus, Leibniz, and Samuel Clarke (to name a few), and it is the argument that Kant labels "cosmological" in the First Critique. I am going to assume the argument that Aquinas develops in chapters 3 and 4 of De Ente et Essentia is an elaboration of the second way. The argument's form is quite simple:
 
1. Some things actually exist (as known primarily by sense perception).
2. Every finite thing (i.e., thing for which there is a real distinction between essence and existence) that actually exists has an actually existing cause of its existence.
3. Causes are prior to their effects in the order of actual existence.
4. Every chain of causes has an essential structure.
5. The essential structure of every chain of causes is finite (has terminal, uncaused nodes).
6. Therefore, there exists at least one actually existing infinite being, and every finite thing is caused by one or more such beings.
7. There can be no more than one infinite being.
8. Therefore, there exists exactly one actually infinite being, which is the ultimate cause of every finite being.
 
If something has actual existence, and this actual existence is something distinct from its essence, then there is some part of the thing's essence which is disjoint from its existence. The finite thing's existence depends on a joining or combining of its existence with the remainder of its essence, and this joining or combining must have some explanation. It cannot be explained by the remainder of the essence, since, it it were, the thing would be a cause of itself (contrary to premise 3). It cannot be caused by the thing's actual existence, since a thing's existence cannot be prior to any part of its essence, since every 'act' or 'event' of existence depends for its very identity on the essence that it actualizes. Hence, the only possible explanation for this joining or combining must appeal some distinct entity (its efficient cause).
 
In contrast, if a thing's existence is identical to its essence, no explanation of its existence is needed or even possible. The essence of a thing constitutes its possible existence, and if a thing's existence is identical to its essence, then that thing's actuality is identical to its possibility. Hence, we cannot sensibly ask how or why its possibility has been actualized. For such a thing to be really possible is for it to be actual, and vice versa. If such a being exists at all, it must exist with absolute necessity. For this reason, the causal principle (premise 2) must be limited to finite things.
 
Premise 7: There can be only one infinite thing (so defined). Suppose that there were two. We can ask whether it is possible for one to exist without the other. If so, then one or the other exists only contingently, which we've have shown to be impossible. So, it must be necessary for both to exist and to relate to each other as distinct entities. Is this relation of distinctness contained in the essence of one or both? It couldn't be just one, since, if it were, the other would depend for its existence on the other. So, the relation must be contained in both essences. But this would introduce some complexity into both essences, which means that we could no longer identify either essence with a simple act of existence. In addition, there would have to be some explanation of the symmetry. Each would have to depend essentially on the existence of another, again contrary to their simplicity and uncausability.
 
This leaves us with premises 4 and 5, which jointly rule out the possibility of an essentially infinite regress.  I will take up this crucial question next time.