Two Kinds of Causal Series
One billiard ball imparts motion to another; an array of falling dominoes, one toppling over another; Abraham begets Isaac, and Isaac in turn begets Jacob; These events share in common a particular mode of causality, namely, that of one member conferring some perfection in a later member — e.g. being or motion — in such a way that the later member now possesses this perfection independently of the first member. Once a billiard ball is set in motion by another, it will continue in motion independently of the activity of the earlier member. Similarly, once Isaac begins to exist, he will continue on existing long after Abraham has passed. Each member is causally dependent on the previous member only initially — that is, during a temporal first moment — after which the relationship of dependency will only be accidental.
Imagine now a causal series where the dependency of every later member upon an earlier member is accidental in the sense outlined above. Once Z is brought into being, it can continue on existing without Y; once Y is brought into being, it can continue on existing without X; and so on and on….to infinity? Absent some proof that an infinite regress of causes is impossible, there is nothing about this type of series which logically demands a first event. Call this a horizontal series of causes.
Unlike horizontal causal series, a vertical series of causes is such that no member in the series could be causally active, here and now, or at any point at which it exists, were it not for the concurrent activity of the other members. In other words, vertical causal series are such that the priority between each member is of a logical kind, and not of a temporal kind. A lake depends, here and now, upon warm air, which in turn depends, here and now, upon heat from the sun, itself dependent, here and now, upon nuclear reactions taking place within the sun; and so on. This chain of dependency takes the structure of a hierarchy, leading to ever deeper, more fundamental levels of explanation. And, like a hierarchy, it must necessarily culminate in a ‘top-level’ cause of the entire series, a cause that is first in the sense of being the most fundamental, that by the sake of which all subordinate causes bear the causal powers that they do.
Consider the following examples:
A: Think of a train pulling a series of freight cars. It is clear that every freight car derives its motion from an earlier member only in a proximate way. Each freight car owes its motion to another freight car which can impart it; but the same applies to that freight car, and so on . There must be something which ultimately explains the power of motion of the entire series; something which imparts motion without deriving its power to do so from anything else. This “something” is the train engine. The train engine is the ‘first cause’ of the entire series, inasmuch as it is the most fundamental cause of the motion of the freight cars.
B: A twister causes great damage to your town. You find a crank radio, and hear an SOS transmission. You assemble a team to find the source of the transmission, which you trace back to a radio tower acting as a repeater. Have you found the source of the transmission? No, you haven’t. The radio tower is doing no more than receive the SOS signal from a previous source, amplifying it, and re-transmitting it. The search continues, and eventually you find the ultimate source of the transmission: a small group of survivors broadcasting from the outskirts of town. Because without a source of the transmission there would be no transmission to be received, one must ultimately arrive at some non-derivative source — namely, the survivor’s broadcast — of which all other members are but carriers.
 Note that the causal relation between each member is not temporal, but logical.