The reason why a hierarchical causal series must terminate in a first member becomes evident once you fully understand the nature of said series and its distinction from an accidental or linear causal series. In a linear series we can trace the activity or existence of each member temporally, backwards in time. For instance, a son is begotten by his father who is in turn begotten by his father. Now each member of this series is not continually dependent on any prior members for its causal activity and existence. The father can die and the son can still go on living, and eventually carry on the linear causal series by having a son of his own. What does this mean? It means, roughly speaking, each member in a linear exists and operates in an independent fashion, not relying on earlier members for its existence or causal efficacy. 

Now in a hierarchical series we are not concerned with the temporal regression of a causal series. Rather, we're concerned with the here and now. The son exists and operates here and now because of his bodily structure, the proper arrangement of his organs, his cellular structure, his molecular structure, atomic structure, subatomic structure. Notice that each of these members in the series cannot exist or operate without continually relying on prior members. The bodily structure would not, here and now, exist or properly function without the existence or proper arrangement of his cellular structure. No member exists or operates independent of the existence or operations of prior members--each member, save for one, is contingent here and now on those that are logically prior to it. What does this mean? It means we have an entirely contingent chain of beings--you could regard the entire chain of these contingent beings, these dependent and derivative beings, as just one contingent thing. What is this chain contingent on? What confers to it the power to exist, operate, to move? It can't be something that is dependent on a prior member for its existence and operation, or else this too would become merely another dependent member of the hierarchical chain. 

So, in the last analysis, in order to explain to existence and operation (or generally speaking, motion) of beings in a hierarchical chain, it is metaphysically necessary to posit a first member who is not contingent (actus purus, ipsum esse subsistens). Why does Thomas not say the same for a linear series? Because each member in a linear series can operate and exist without the continual existence or operation of any prior members. It is not existentially and causally impoverished like the hierarchical series. That being said, each member of a linear series presupposes the more fundamental hierarchical series  for there to even be a linear series. Hope this makes sense. Sorry, I think I rambled a bit. 

Last edited by RomanJoe (2/10/2018 10:53 pm)