By Joseph M. Magee, Ph.D.

After presenting the “first and more manifest way” of proving the existence of God by reason alone (without the aid of God revealing himself in Sacred Scriptures), in Summa Theologiae Ia, 2, 3, Saint Thomas Aquinas continues this project by turning in the “Second Way” to what he somewhat enigmatically calls “the nature of the efficient cause.” The greatest obstacle to understanding his Second Way, though, is determining precisely what Aquinas means by “the nature of the efficient cause” and “an order of efficient causes,” and how the Second Way is distinct from the First and Third Ways. For all three make use of efficient causes: motion, i.e., accidental change, in the case of the First, or generation and corruption, i.e., substantial change, in the Third. It seems unnecessary for Aquinas to use efficient causality in a general sense as the basis of a proof distinct from these two species of it.

Given that Aquinas is writing his summary of theology for beginners in the field, it seems a reasonable assumption that he will not pursue redundant or superfluous arguments. Accordingly, it seems that each ‘way’ of proving God’s existence embodies and employs a distinct perspective on the physical world of which Aquinas pursues an ultimate explanation in God. The particular five proofs are based on different aspects of reality that Aquinas believes are apparent to an intelligent observer, aspects whose ultimate explanation is God. They begin with the observation of motion, efficient causality, contingent beings, grades of perfection and goal seeking behavior. But more than simply considering these phenomena as effects, or things to be explained (explananda), each ‘way’ argues in terms of an intelligible character or ratio that is the proper domain of each proof. In a generic sense, particular substances in the physical, sensible world are the common starting point for each distinct proof. But Aquinas does not simply argue for an ultimate cause of all sensible material things taken as an undistinguished whole. Rather, he examines physical things (things in matter and motion) in terms of, or under the intelligibility of, different types of causality.

Motion/alteration (accidental change) (First Way) is clearly distinct from generation/corruption (substantial change) (Third Way) which are themselves clearly distinct from nobility/perfection in being (Fourth Way), which again are each distinct from end-directedness (Fifth Way). What is not clear is how motion/alteration on the one hand, and generation/corruption on the other, are each distinct from efficient causality in general so that the latter can be the basis of a distinct proof (Second Way), as both seem to be species of the latter. Indeed, several commentators are content to view the Second Way as a generalized version of arguments specified in the First and Third Way.

For instance, Christopher Martin in Thomas Aquinas: God and Explanations states “[T]here is a good case for making out that the Second Way is a generalization of the First. The First Way has to do with process of change, the Second Way has to do with efficient causality in general.” Martin nevertheless goes on to consider that the Second Way may seek to explain a thing’s being, in which case the first two ways would be distinguished from each other, for “[a] beginning of existence would thus fall under the Second Way, but not under the First Way” (p. 146). In considering this latter possibility, he echoes other contemporary expositors of the Second Way.

To the extent that commentators see the need to clearly distinguish the First, Second and Third Ways, the most common view is to see the Second Way as focusing on the efficient causing of the existence of substances, i.e., their coming-to-be or generation, while the Third Way examines the continuation-in-being of possible beings as (efficiently) caused by necessary being(s). This existential reading of the Second Way finds an early champion in Etienne Gilson, who in The Christian Philosophy of St. Thomas Aquinas says that Aquinas was influenced by Avicenna to go beyond Aristotle in Metaphysics II, 2 (994a1-19) to discover a “distinction, which the second way presupposes, between the causality by way of motion and efficient causality, whose proper effect is Being” (p. 66). Gilson himself contends that texts of Avicenna supporting the discovery of this novel distinction shaped Aquinas’ reading of Aristotle, texts which he notes are found in Clemens Baeumker, Witelo, ein Philosoph und Naturforscher des XIII Jahrhunderts, (Munster 1908) p. 328-330 specifically, Avicenna’s Metaphysics, tr. 8, c.1-3.

G. E. M. Anscombe and P. T. Geach, in their presentation of the Five Ways in the section on Aquinas in Three Philosophers similarly declare, “The first two ‘ways’ differ only in that one relates to processes of change and the other to things’ coming to be; the further argument is quite parallel in each case” (p. 113). John F. Wippel in “The Five Ways” in Thomas Aquinas: Contemporary Philosophical Essays, basically adopts an interpretation of efficient causality in the Second Way as focused on being, but nevertheless sees it as also applicable to the First: “Although [Aquinas] does not spell out particular examples for us in this argument, he would undoubtedly include substantial changes (generation and corruption of substances) as well as various instances of motion taken strictly which we have discussed above in connection with the first way, i.e., alteration, local motion, increase and decrease” (p. 171-2).

William Lane Craig in The Cosmological Argument from Plato to Leibniz, accordingly credits Gilson with promoting this view of the Second Way focused on efficient causes of being. “Gilson maintains that Aquinas in the second way is contemplating a series of agent causes which produce, not just change, but the very being of their effects” (p. 176-7). Craig believes that in Aquinas’s understanding of the world, efficient causality of being in the sensible world depended on Ptolemaic spheres, yet nevertheless, Craig believes we can find

something which is of value when the proof is divested of its medieval trappings. A sympathetic reformulation of Thomas’s second way might suggest, for example, that my existence now is dependent upon the temperature of the earth’s atmosphere, which in turn is dependent upon the distance of the earth’s orbit from the sun, which is dependent upon the mass of the sun, which is dependent upon the sun’s relation to other stars, which are dependent for their existence upon our galaxy, which is dependent for its existence upon surrounding galaxies, and so on and on into the recesses of the universe.” (p. 177)

Without noting the controversy, Brian Davies in The Thought of Thomas Aquinas, simply asserts, “According to the Second Way, there are causes in the world which bring it about that other things come to be” (p. 29). Similarly, Edward Feser in Aquinas: A Beginner's Guide, agrees with Craig that Gilson is the source for reading the efficient causality of the Second Way in terms of causes of being, and he believes that this is what distinguishes the Second from the First Way.

A more plausible and interesting account of the difference between the arguments is provided by Etienne Gilson, who suggests that whereas the First Way is concerned to explain why things undergo change, the Second Way is intended to explain why they exist at all, where (as in the First Way) the causal influence of the first cause is not something that occurred merely at some point in the past, but which exists here and now. (p. 86)

The main textual reason within the Second Way in support of this existential reading seems to be that the Second Way contains the premise, “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.” These commentators take Aquinas’s use of ‘it would be prior to itself’ (esset prius seipso) to mean ‘existing prior to itself’ such that a thing would have to be a distinctly existing substance prior to itself in time, in order to bring itself into existence, which would, of course, be impossible. Because this text is virtually identical to what Aquinas says in Summa Contra Gentiles I.13 where Aquinas cites Aristotle in II Metaphys (par. 33 Procedit autem), Gilson reasonably supposes that the Second Way likewise derives from this Aristotelian text as well (p. 66). But Gilson reads both texts in the existential sense and so sees this a significant departure from Aristotle and as evidence of the influence of Avicenna (p. 68). However, the context of SCG 1.13 does not demand, and perhaps even militates against, an existential reading of efficient causes there, for it is introduced as addressing whether self-movers (souls) require “a first mover immovable and separate” (SCG 1.13 par. 32 Et ad hoc). And, Aquinas’ own commentary on Aristotle’s Metaphysics II, 2 considers the impossibility of an infinite series or ordered efficient causes only in terms of causes of motion, not in terms of causes of existence

Second, he [Aristotle] gives an example of this in the genus of efficient cause. He says that it is impossible to have an infinite series in the genus of cause which we define as the source of motion—for example, when we say that a man is moved to put aside his clothing because the air becomes warm, the air having been heated in turn by the sun, the sun having been moved by something else, and so on to infinity. (In II Meta., lect. 2, no. 300 Exponit autem)

Despite the wide acceptance of the existential reading of the Second Way, Aquinas’ other deployments of this argument against an infinite regress in efficient causality do not support such a reading, and instead situate it in the context of the operations of animate movers.

There are also at least three reasons drawn from the text of ST Ia, 2, 3, to resist this metaphysical reading. First, if the Second Way were starting with the efficient causation of substances in being, the natural focus would be on their coming to be in time, as a father is begotten by his parents, and begets in turn his children. But this is just to envision a per accidens series of efficient causes, of which a later premise in the argument (“to take away the cause is to take away the effect”) is not true. The grandfather can die, without either the son or the granddaughter ceasing to be. What is needed for that premise to be useful is for the series of efficient causes to be ordered per se, i.e., as a series of essentially subordinated causes (which ordering will be further explained anon). This per se ordering of efficient causes of being does not seem evident to the senses (what “we find in the world of sense” (invenimus enim in istis sensibilibus)), but requires adopting a metaphysical perspective. Indeed, Feser urges that this metaphysical perspective as found in Aquinas’s On Being and Essence (De ente et essentia), chapter 4, illumines the sense of efficient causality Aquinas has in mind in the Second Way. Wippel, however, notes the non-intuitiveness of this metaphysical perspective. One can find, he says, “a more metaphysically grounded illustration of efficient causality at work” which “Thomas has developed … in c. 4 of his De ente et essentia. However, I hasten to add that this is not the kind of efficient causality that is immediately given to us in sense experience” (p. 172).

The second reason to resist reading the Second Way metaphysically is that Aquinas’s statement against self-causation does not require a reading focused on the causation of being. The priority at issue (nothing could be the efficient cause of itself, for then it would be prior to itself) can equally apply to motion or other simultaneous sustaining efficient causes, not only to coming to be in time. It would be just as true for Aquinas to say, “There is no known case…” of a moving cause which “is found to be the efficient (moving) cause of itself; for so it would be (simultaneously, essentially and per se) prior to itself; which is impossible.” That is, it would be equally impossible that a turning cog could be the prior simultaneous moving cause of its own turning, as it would be for anything to be the temporally prior cause of its own manufacture or generation. So, this premise is no reason to suppose that the Second Way is concerned with the coming into being or generation of things, but it is perfectly compatible with the reading of it I will propose.

Finally, generation and corruption are explicitly given as the starting point of the Third Way. There Aquinas explains how we know there are possible beings: “We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt.” And so, if coming into being of substances is the focus of the Second Way, this, again, would render either it or the Third Way redundant and superfluous.

Fortunately for us who believe there must be a particular type of causality that Aquinas had in mind in delineating a Second Way in contradistinction to his First and Third Way, motion/alteration (accidental change) and generation/corruption (substantial change) are not the only two types of efficient causality for Aquinas to build his arguments on. The proper effect Aquinas employs in the First Way is the most obvious sense of motion or physical change. Elsewhere he calls this kind of change transient alterations: the successive replacement of one accident by another. It includes physical displacement where an object’s position alters by occupying one place after another (what modern physics means by motion) as well as other material changes of accidents as when one color, shape, quantity, etc. succeeds another successively (through a certain duration of time). Aquinas, however, distinguishes these types of alterations from immanent immutations or activities (also called ‘operations’) whereby cognitive faculties (sense and reason) and appetitive powers (sense desire and will) become actual. This kind of ‘alteration’ does not occur by one accident gradually succeeding another over time, but all at once, with the accident being received or present formally (as opposed to materially). Aristotle in De Anima 2, 5 calls it “a preservation of that which is potential by something actual which is like it” and is different from alteration which is “the destruction of something by its contrary” (417b2-8 (See also, Thomas Aquinas, Commentary on Aristotle's De Anima, Book II, Lecture 24, nn. 551-555.)) Aquinas describes this type of ‘alteration’ in the Summa Theologiae in these terms:

Now, immutation is of two kinds, one natural, the other spiritual. Natural immutation takes place by the form of the immuter being received according to its natural existence, into the thing immuted, as heat is received into the thing heated. Whereas spiritual immutation takes place by the form of the immuter being received, according to a spiritual mode of existence, into the thing immuted, as the form of color is received into the pupil which does not thereby become colored. Now, for the operation of the senses, a spiritual immutation is required, whereby an intention of the sensible form is effected in the sensile organ. Otherwise, if a natural immutation alone sufficed for the sense’s action, all natural bodies would feel when they undergo alteration. (ST I 78, 3)

The Second Way, in fact, focuses on the efficient causality at work in operations or immanent activities; this provides the distinctive ratio or type of causality that is the starting point and perspective on sensible, material reality for a Way distinct from the First and Third Ways.

There are at least three reasons that support reading the efficient causality that is the focus of the Second Way as pertaining to immanent alterations or operations within the soul, the first of which I have just outlined, namely that it gives the most coherent way to distinguish the Second Way from both the First and Third. In order for all three to focus on different types of efficient causality, cognitive/appetitive operation provides a type of efficient causality distinct from alterations or accidental change (motion) on the one hand, and substantial change (generation/corruption) on the other.

The second reason is found in the First Way, where at a key point in that proof, Aquinas introduces the remaining line of reasoning with a conditional.

If that by which it (a moving thing) is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again.

In the First Way, Aquinas is developing his argument by considering only the case when the prior actuality causing motion is itself a motion or a process of change. The fact that he presents this focus as the antecedent of a conditional proposition seems to indicate that there is another proper actuality or type of causality that can produce motions (transient alterations) that is not itself a motion or process of change. I suggest that this other type of actuality and efficient causality would be immanent activities, and so it is the focus of another proof (the Second Way).

In his exposition of the First Way, John Wippel notes that initial opposition to the argument focused especially on rational agents as counterexamples that prevented ascribing a cause of all motion to God as First Immobile Mover.

Many of the objections against the argument [the First Way] have to do with its claim that whatever is moved is moved by something else. Already within Thomas’s century there were those who denied that this applies to all cases. Exceptions should be made, it was argued, and first and foremost for spiritual activities such as human volition. Not long after Thomas’s death Henry of Ghent maintained that a freely acting agent can reduce itself from a state of not acting to acting, or as he would eventually put it, from virtual act to formal act. (p. 163)

Wippel notes, however, that for Aquinas, acts of volition are no counterexamples since these, too, must have God as their ultimate (efficient) cause. According to ST I-II, q. 9, a. 4 (parallel in De Malo q. 6), Aquinas argues that “what first moves the will and the intellect must be something above them both, i.e., God.”

[I]nsofar as the will is moved to exercise its act of willing, we must also hold that it is moved by some external principle. That which is at times an agent in act and at times an agent in potency must be moved to act by some mover. Such is true of the will. … Thomas concludes that the will proceeds to its first motion from an impulse (instinctus) given it by some external mover, and finds support for this in Aristotle’s Eudemian Ethics (VII, c. 14 (1248a 25-32)).

In his Disputed Questions de Malo, q. 6, Thomas develops much of the same thinking. … Thomas again concludes the will is moved to its first motion by something external or exterior to it. He goes on to tell us more about this externally caused impulse which first moves the will to act. This impulse cannot be provided by the heavenly body, since the will is grounded in reason (the intellect), which is not a corporeal power. He concludes, therefore, developing a remark made by Aristotle in his De bona fortuna (really taken from Bk VII of the Eudemian Ethics as he indicates in ST I-II q. 9), that what first moves the will and the intellect must be something above them both, i.e., God. Since God moves all things in accord with their nature as movable beings – for instance, light things upward and heavy things downward – so does he move the will in accord with its condition or nature, that is to say, not in necessary fashion but as undetermined or freely. (165-6)

I would contend that Wippel’s analysis of acts of willing as conforming to the general principle that whatever is moved is moved by something else is, in fact, correct and in accord with Aquinas’s thought. Aquinas, however, in the First Way is only interested in motion that is the result of prior motions (transient alterations), and so, by means of his conditional clause, relegates the causal efficacy of volitional acts to a consideration that was beyond the scope of the First Way, namely the Second Way. It seems Wippel, too, recognizes that immanent activities go beyond the scope of the First Way for

… in his [Thomas’s] eyes even free human activity does not violate his conviction that whatever is moved is moved by something else. At the same time, I would not recommend that one take human volition as the point of departure for Thomas’s argument from motion for God’s existence. This does not seem to be the kind of starting point he has in mind for his “more manifest” way, since some philosophical effort will be required to show that motion by something else is involved in volition. (p. 167)

Indeed, Wippel seems to believe that immanent activity, at least of movements of the will, must have a cause in something other than the creature that is the subject of such activity. He also seems to think the knowledge of this causal dependence is consequent on demonstrating God’s existence (through other types of causality), so it cannot serve as the basis for such a demonstration. “Moreover, the full appreciation of Thomas’s views concerning the interrelationship between divine causal activity (as the first moving cause) and free human activity presupposes that one has already demonstrated God’s existence and identified him as a creating, a conserving, and the first moving cause” (p. 167).

Given inherent causal dependence of acts of volition owing to their variability, I fail to see any justification for ruling out immanent activity as a basis for a demonstration that God exists. If, as Aquinas believes, such activity must have an efficient cause, and given its nature as non-transient, it must be caused per se in an essentially ordered series. And as he argues in both the First and Third Ways, as well as in the Second Way itself, such a series cannot be infinite, a first cause for the observed last effect of volitional operation must necessarily exist. It seems, then, that immanent activities (at least of the exercise of choice) would eminently serve as the basis for a proof of God’s existence.

The final reason to believe that the Second Way has immanent activities or operations of the will as the kind of efficient causality as its focus is how Aquinas answers the Second Objection of this article, ST I, q. 2, a. 3. The objection contends that it is not necessary to posit God’s existence to account for the world since everything can be explained either by nature or by human volition. Aquinas’s response with regard to nature clearly refers to the Fifth Way:

Since nature works for a determinate end under the direction of a higher agent, whatever is done by nature must needs be traced back to God, as to its first cause.

But his response with regard to human volition anticipates his contention that God is necessary to explain the variability of acts of willing (as cited by Wippel above, ST I-II, q. 9, a. 4 and De Malo q. 6).

So also whatever is done voluntarily must also be traced back to some higher cause other than human reason or will, since these can change or fail; for all things that are changeable and capable of defect must be traced back to an immovable and self-necessary first principle, as was shown in the body of the Article.

In the body of the article, however, Aquinas has not explicitly shown that God, as a higher cause, immovable and self-necessary first principle is necessary to account for the change and failure of human reason or will, unless it is in the Second Way, understood as focusing on the actuality and causality of operations, as I have been arguing. He has shown that such an immovable first principle is required to explain transient motion in the First Way, while leaving out any other kind of efficiently caused actuality that might serve to cause such motion. And he has also shown in the Third Way that the reality of changeable contingent beings capable of defect must be traced back to a self-necessary first principle. What seems left out by these two ways of tracing back sensible realities to God through a series of causes is ‘whatever is done voluntarily’ (except incidentally inasmuch as such actions depend on the reality of contingent beings). If the Second Way is concerned with the efficient causing of immanent activities, however, it would also show the need for an immoveable first principle to explain this sort of variability and defect explicitly raised as an objection to proof of God’s existence.

Dr. Joseph Magee earned his PhD from the Center for Thomistic Studies at the University of Saint Thomas, Houston, Texas, and regularly teaches philosophy of religion at the Sam Houston State University. He produces and maintains the Thomistic Philosophy Page.