17. The short form of the Second Way is as follows:
A. We observe in things an order among efficient causes.
B. In any series of efficient causes, the ultimate effect depends on the intermediate causes, which depend in turn on the first cause.
C. To remove the first cause from a series of efficient causes is to deprive all the subsequent causes of their efficacy.
D. In an infinite series of efficient causes, the first cause is removed.
E. Consequently were the order of efficient causes to extend to infinity, there would be no change at all.
F. There must therefore be a first efficient cause.
18. In reflecting on the Second Way, two difficulties present themselves. First, we must distinguish the reasoning given here from the reasoning given in the First Way. Second, we must explain how this argument leaves open the philosophical conceivability (defended by St. Thomas in Ia q.46 a.2) of a universe without beginning.
19. The second way differs from the first in that where the first way concerns itself specifically with the nature of motion and the principle that everything moved is moved by another, the second way concerns itself specifically with the properties of ordered efficient causation. (An efficient cause is an agent which brings about a change in something.)
20. In examining the first way, it was important to note that the series of movers under consideration is simultaneous and not successive. Here again, we should not assume that St. Thomas has in mind a purely successive chain of efficient causes. If this were the case, his assumption would contradict the point he is trying to prove, since his first efficient cause would merely be the first event in a historical series. Rather, St. Thomas wants us to grasp that the order of efficient causes extends beyond any set of temporal agents to something which gives nature and agency to everything else.
21. The argument, then, can be reformulated as follows: It is evident in experience that there is some order among efficient causes: the coming to be of certain things is brought about by other things, and those things in turn are made to act by still prior agents. Since the efficacy of any series of intermediate causes depends on the efficacy of the first cause in setting them all into action, there must be some absolutely first cause, which lends efficacy to the intermediate causes which make up the world and bring about all the effects observed in our experience. Were there not a first cause, we could follow the chain of efficient causes ad infinitum, perpetually deferring the answer to the question "what is the basis of the change of all these things", since each object supplied to answer the question would itself beg the same question further up the chain, leaving a brute fact or void of causation for us to confront. Thus we grant the existence of a cause which is utterly first in the order of efficient causes.