Causality and St. Thomas Aquinas’ Second Way



The second way of proving the existence of God by St. Thomas Aquinas is that of efficient causality. It is not just any old efficient causality such as that of local motion, which is observable through human sensation. It is efficient causality at the level of existence, the level of divine creation.

The Second Way: An Apparent Emphasis on Serial Efficient Causality

It could be argued that the conclusion of a recent essay of mine in Catholic Stand contradicts St. Thomas Aquinas’ Second Way of proving the existence of God. I indicated that causal sequences are irrelevant. Yet, the Second Way in the Summa Theologiae reads,

The second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God. (Summa Theologiae I, Q. 2, A. 3 resp.)

St. Thomas’s Earlier Emphasis on Existential Efficient Causality

It appears as if the argument form efficient causality is primarily based on causal sequences, but it is not. St. Thomas had earlier presented the argument from efficient causality as based on the existential flaw in every created entity in that its act of existence is really distinct from its nature. He argued that the efficient cause of the existence of every such entity must be a unique being whose nature is solely its act of existing. That unique being is God.

But it is impossible that the act of existing of a thing be caused by a thing’s form or its quiddity, (I say caused as by an efficient cause); for then something would be the cause of itself and would bring itself into existence — which is impossible. Everything, then, which is such that its act of existing is other than its nature must have its act of existing from something else. And since every being which exists through another is reduced, as to its first cause, to one existing in virtue of itself, there must be some being which is the cause of the existing of all things because it itself is the act of existing alone. If that were not so, we would proceed to infinity among causes, since, as we have said, every being which is not the act of existing alone, has a cause of its existence. (On Being and Essence, translation by Armand Maurer, p. 47)

Immediacy vs. Intermediacy

From this argument, it is evident that there can be no intermediary cause of existence between the I AM, who is the efficient cause, and the effect, the existence of a creature. God as the First Efficient Cause of each creature is first in the sense of immediate and sole, not in the sense of serial enumeration. If there was even one intermediate, it would be exercising divine power, which is impossible.

Also, the alleged serial causal sequences proposed by several apologists, are not based on the nature of material reality as God created it, but on human artifacts such as trains and chains.

My Catholic Stand colleague Anthony S. Layne mentioned to me that, in the quotation above from the Summa, St. Thomas notes that it is in the world of sense in which it appears to us that there are natural causal sequences. Indeed we know of causality through our sense knowledge of material things. Materiality, as we know it, is subject to change and as such is serial. Even our thought processes are rational, i.e. serial. This is in contrast to intuitive knowledge which perceives reality as a whole and not in the bits or stages of human reason.

Real vs. Logical Distinctions

St. Thomas’s argument for the existence of God as First Efficient Cause in On Being and Essence notes that there is a real distinction between nature and existence in created entities, whereas there is no real distinction—only a logical distinction—between the nature and existence of God.

If the argument from efficient causality as presented in Part 1, Question 2, Article 3 of the Summa Theologiae identified God as the numerical first efficient cause of a hierarchical or linear series, it would be implicitly denying a distinction in nature between God and creation at the level of existence. A numerical first implies no difference in kind. This could take either of two forms. One form would be that of pantheism, in which God is simply the first of a material sequence. God would be numerically the first efficient cause at the level of material reality, in modern parlance, the Big Bang. This would bring God down to the material order. The other form would elevate material reality to divinity. Material reality would simply be a series of emanations of God. Rather than being really distinct in its existence from God, material creation would simply be a serial epiphany of a singularly divine reality.

The argument from efficient causality, as St. Thomas presents it in On Being and Essence, clearly identifies a real distinction between nature and existence in each created entity, which necessitates the existence of a unique being in whom nature and existence are identical and thereby only logically distinct. This also necessitates a real distinction between God and creature.


The argument from efficient causality, as St. Thomas presents it as the Second Way in the Summa Theologiae, does not clearly distinguish between serial efficient causality at the material level of local motion and existential efficient causality, which is an immediate act of divinity. It is from this lack of clarity that irrelevant analogies to the serial relationship among the artifacts of trains and chains arise. Nevertheless, the Second Way of the Summa Theologiae is properly understood as in full accord with On Being and Essence, which was written by St. Thomas, earlier in his academic career than the Summa. They certainly differ in emphasis.

For other aspects of the contrast between the argument from serial efficient causality and the argument from existential efficient causality see “Is God, as First Cause, a Numerical First of Many Causes?

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