​                                                                             by    Brother Thomas Aquinas

Near the beginning of the Summa Theologiæ, St. Thomas Aquinas provides five arguments for God’s existence that rely on human reason without recourse to articles of faith or revelation. Since the Summa is by title a summary, Aquinas’ presentation of the “Five Ways” is rather terse, but the philosophical details are clarified in other writings, both by the saint himself and by subsequent authors. Nevertheless, the abbreviated versions are valuable in their own right for one wishing to study the Catholic faith and all it encompasses.
 
In the second of the Five Ways, which we consider here, St. Thomas argues from the principle of causation, and his reasoning may be expressed as follows:

1.  We observe in the world effects brought about by causes.
2.  In some cases, the effect of one cause is itself the cause of a subsequent cause.
3.  Although every effect is the result of some cause, nothing is the cause of itself, for this would require it to precede itself in the order of causation.
4.  In the absence of any particular cause, neither does the corresponding effect occur.
5.  A sequence of causes cannot extend backward infinitely, for then there would be no first cause to bring about any of the others or the ultimate effect.
6.  But we do observe sequences of causes and effects, so there must be a first cause, which we call God.
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Though many have challenged this argument in varying ways, it is the fifth point that seems to be the least understood. Indeed, there are two different ways to build a sequence of causes and effects, and the distinction between these ways is of crucial importance to the syllogism as a whole.
 
When discussing sequences of causes that may or may not “extend backward infinitely,” there is a strong tendency to envision a sequence that extends backward in time: a line of dominoes falls by successive collisions of one domino into the next. However, these are examples of what metaphysicians call accidental sequences: once a particular cause has produced its effect, it is no longer needed for any subsequent effects. A man can beget a child even though his parents have died. Since any particular cause in an accidental sequence depends upon only the prior cause and is necessary for only the subsequent effect, there is no theoretical reason such a sequence could not extend infinitely far into the past. As long as each domino knocks into the next, the line will continue to fall.
 
Aquinas’ argument fails if the sequences in question are accidentally ordered, but this is not what he had in mind. Instead, the Second Way appeals to essentially ordered sequences of causes. In an essentially ordered sequence, each cause depends from moment to moment on all prior causes, and in fact all causes exist and act simultaneously in a hierarchical structure. For example, an ornate chandelier is suspended above a foyer not only by the first link, to which it is directly connected, but also by every other link in the chain and, ultimately, the fixture on the ceiling. If any link breaks, the chandelier will fall.
 
The utility of this analogy extends even further. Notice that an infinitely long chain is useless to hold the chandelier without the force of a ceiling fixture. Indeed, although each link can be proximally explained by reference to the next higher link, we must eventually reach an agent with its own explanatory power. We achieve nothing by indefinitely deferring such an explanation.
 
The distinction between accidentally and essentially ordered sequences of causes is tantamount to a proper interpretation of Aquinas’ argument. Equally important, however, is the choice of analogy used to illustrate the argument. Therefore, against the multitude of ineffective examples, it will be profitable to identify another that is useful: given an exceptionally long train of boxcars rolling up an incline, we can conclude, even without directly observing it, the existence of a locomotive at the front (or, at least, some external force) that is ultimately responsible for the motion, since boxcars cannot move themselves.
 
It is in this sense—that of boxcars, rather than dominoes—that Aquinas intended his Second Way be read, and doing so establishes the necessity for a first cause, namely God, in order to satisfactorily explain the causes and effects we observe here and now. In fact, since it appeals to essentially ordered sequences, rather than accidentally ordered ones, the argument would be just as effective in a universe that had always existed. And with similar diligence to distinguish the precise meaning of Aquinas’ terminology, there is much to be gleaned from his other writings as well.