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Understanding the Mysterious Fifth Way to God’s Existence

The fifth way is taken from the governance of things. For we see that things which lack knowledge, such as natural bodies, act for an end, which is apparent from this: that always, or more frequently, they act in the same way, so as to obtain that which is best. Hence it is plain that they achieve their end, not by chance, but from intention. However, those things which do not have knowledge do not tend toward an end unless directed by something with knowledge and intelligence -- as the arrow [is directed by] the archer. Therefore, there exists some intelligent being by whom all natural things are directed to an end: and this we call God.
Summa Theologiae, I, q. 2, a. 3, c.  (Leonine edition, translation mine)

The Quinta Via's Setting

St. Thomas Aquinas wrote his five ways to God’s existence in the very first pages of his Summa Theologiae (1265-1274), the finest, most mature synthesis of his philosophical and theological thought – a work designed both for educated laity and seminarians. Since God’s existence is the foundation on which logically rests the entirety of his multiple volume masterpiece, giving but a short paragraph’s treatment to each “way” clearly signifies that no complete scholarly demonstration was ever intended. Rather, the “ways” are merely short summaries of St. Thomas’ take on classical arguments his students already knew well.

Hence, expecting fully developed philosophical proofs in the five ways is a major error.

Crucially, the quinta via (fifth way) is not an argument from design, like that of William Paley (1743-1805), who reasoned from extrinsic finality that, like a watch, the world exhibits deliberate design because of perfect coordination of its parts. Rather, St. Thomas argues from intrinsic finality that all natural bodies lacking knowledge act for an end, thereby revealing that they are moved by an intelligent agent, whom we call God.

St. Thomas maintains that natural bodies act for an end “so as to obtain that which is best” because they are moved by natural appetite. Since he maintains (1) that natural appetite seeks what is fitting to a thing and (2) that what is fitting to a thing perfects it, it follows that natural bodies are acting for “that which is best.”1 Nonetheless, maintaining that natural bodies attain “that which is best” is not essential to his argument, since, as will be shown, it is rationally demonstrable that natural bodies attaining merely definite ends require an intelligent director.

Every Agent Must Act for an End

Central to the quinta via is the principle of final causality, which entails two distinct claims, namely, (1) that every agent must act for an end, and (2) that there must be pre-existing intellectual knowledge of the end. This latter claim is the most mysterious one made by Thomistic metaphysicians regarding the fifth way – a claim without which the argument fails to attain any significant traction. Conversely, successful defense of both aspects of final causality proves why it cannot be “explained” as just some form of efficient causality.

An agent is anything that does something, produces an effect. It matters not whether agents are considered macroscopic wholes, like an animal, or just subatomic particles regulated by physical laws. Either way, things appear to act regularly the same way, unless something impedes their action. An example would be classification of chemical elements according to behavior. Were these not consistent in activity, natural science would become unintelligible chaos. Nonetheless, regularity of behavior is not essential to prove the need for an intelligent director. Merely showing that every agent must act for a definite end suffices.

That every agent must act for an end is demonstrable through the principle of sufficient reason. Since agents of a given nature always tend to a certain result or end, there must be a sufficient reason for such regularity. Yet, even were the end not attained regularly, a sufficient reason would still be needed to explain why a certain definite end is achieved as opposed to any other.

The Angelic Doctor explains the role of intention in agents moving toward an end in his Summa Theologiae, I-II, q. 1, a. 2, c:

“But an agent does not move except from an intention to an end. For if the agent were not determined to a certain effect, it would not do this rather than that: therefore, in order that it produce a determinate effect, it is necessary that it be determined to a certain one, which has the nature [rationem] of an end. This determination, as in the rational nature, would be the “rational appetite,” which is called the will; so, in other things, it would be through natural inclination, which is called the "natural appetite."2

If an agent were totally indifferent to multiple possible effects, no sufficient reason would explain why a specific outcome occurs, making production of a definite effect impossible.  But every agent produces a definite effect. Hence, every agent must act toward a definite end.3

Because St. Thomas views chance as an event happening outside the intention (even understood as natural appetite) of an agent, he maintains that chance presupposes intention to an end. Thus, he sees chance as no threat to final causality.4

Moreover, if a reason determines why a specific end comes to be, it must act on the agent from the inception of its agency.

The text cited above refers to things with rational natures. It also refers to “other things,” wherein a determination ”to a certain effect“ is caused by their “natural inclination, which is called the ‘natural appetite.’”5

Here St. Thomas maintains that non-knowing agents cannot act solely by themselves so as to attain a determinate end: “But those things that lack reason tend to an end by natural inclination, as if moved by another and not by themselves: since they do not know the nature of an end [as an end], and thus, they are able to ordain nothing to an end, but can be ordained to an end solely by another.”6

Things lacking reason cannot explain how they attain their ends through their own causality alone, “since they do not know the nature of an end [as an end], and thus, they are able to ordain nothing to an end.” This last clause logically implies that the property of being able to ordain anything to an end necessarily implies the ability to “know the nature of an end [as an end].” Thus, irrational agents must tend to an end “as directed or led by another”, whom St. Thomas maintains is God himself.7

There Must Be Intellectual Knowledge of the End

The most mysterious part of final causality is this claim that even non-knowing agents must be directed to their ends by some intellectual agent. St. Thomas makes this evident when he says that nothing can ordain anything to an end, unless it can “know the nature of an end [as an end].”

This text is critical, since it shows that St. Thomas insists on there being intellectual knowledge of the end, not because of “regularity” in attaining an end, but simply because of the need to know the nature of an end as an endfor to know the end as an end is to know it abstractly, which entails intellectual apprehension.

The dictum that what is first in the order of intention is last in the order of execution8 means, not that the end exists in extramental reality before it is caused by the efficient cause, but rather that the end exists as intellectually known before the agency of the efficient cause can take place. Paradoxically expressed, “the end must exist before it exists.” But “existence” must be understood in two senses: (1) in extramental reality, and (2) in intramental reality, that is, as known by an intellect. It is in the latter sense that the end is the first of all causes, the cause of all causes.

Thus, the proper meaning of “the end exists before it exists” is that the end must exist intramentally before it can exist extramentally.

As will be shown below, this explanation applies even to non-knowing natural bodies.

Philosopher Jacques Maritain, a leading contemporary Thomist, argues that final causality entails intellectual knowledge of the end. In his Preface to Metaphysics, Maritain points to the “relation of the agent to its action, an action distinct from itself.”9 He considers hydrogen and oxygen which are determined to interact so as to produce water, which manifests a real relation of their essence to making water. “To be determined to a term presupposes an ordination, a relation to that term.”10 The term, in the case of hydrogen and oxygen, is the effect of their union, namely, dihydrogen oxide or water. So, the relation entails (1) the hydrogen and oxygen as separate elements, and (2) the product of their union: water. Maritain then follows the logic to its inexorable conclusion:

“… How can there be a relation, an ordination between two things which do not exist in any fashion, or between a thing that exists and a thing which does not? For a relation or ordination to exist between two terms both terms must exist. Therefore an effect of an action must somehow exist if the agent is to be determined, ordained or inclined toward it. What does this mean? It means that the action or effect must exist before it is produced or realized.
But how in the name of heaven is this possible? Only if the action or effect exists as present in thought, with the existence of knowledge. Only in this way can it exist – in thought – before it exists in reality.”11

Maritain summarizes his demonstration, tying its force back to the principle of sufficient reason:

“We see, therefore, that the sufficient reason for an agent’s action, that which determines it to a particular action or effect rather than any other is the effect, the action itself – not as produced and accomplished, but as that which is to be produced, accomplished and therefore as preconceived by a thought, so as to preordain the agent to that action.”12

From this preeminent metaphysician’s proof, it is evident why St. Thomas insists that things lacking reason cannot explain how they attain their ends through their own causality alone, “since they do not know the nature of an end [as an end], and thus, they are able to ordain nothing to an end.”13

Therefore, the complete principle of final causality – a universal metaphysical principle applicable to all agents, intellectual or not – is as follows: Every agent must act for a determinate end, and that end must be intellectually known prior to the agent’s action that produces the end in reality.

Nor need this intellectual knowledge of the end be had exclusively in the case of rational creatures, such as human beings. For, as St. Thomas points out, such intellectual knowledge must also obtain in the case of things lacking knowledge, such as natural bodies. In this latter case, he tells us that the intellectual knowledge is had by God, who directs all things to their proper ends by means of the divine governance. Such is the line of reasoning put forth in the quinta via.

One of Many Intelligent Governors?

Regardless of whether one views “natural bodies” as subatomic entities or as the macroscopic wholes that common sense affirms, the vast majority of such agents lack rational natures. This logically entails that one or more intelligent causes must direct or govern such natural bodies to their proper ends. Since the quinta via is actually an argument from governance of the world, the ultimate question is whether or not all this directed agency must be ascribed to a single intelligent being “by whom all natural things are directed to an end.”14

St. Thomas makes no explicit attempt to prove that there is but a single intelligent governor of all natural bodies in the fifth way. He merely asserts it. Still, later in the Summa Theologiae, he does give an argument for the unicity of God based on (1) the evident unity of the cosmos and (2) the principle that “things that are diverse do not come together in one order unless they are so ordered by one being.”15

Deeper Metaphysics

When a natural body is moved by its “natural appetite” to a certain end, the end may be conceived as anything broadly in keeping with the activities of that agent’s nature. A rock rolling down a hill might be thought to fulfill its end merely by reaching any lower level. But, following Maritain’s reasoning, it is not just some “broadly conceived end” that constitutes the pre-known terminus. Rather, it must be the “exact end as actually achieved” that is pre-known, since that unique reality is one of the two terms involved in the action.

As Maritain observes, “For a relation or ordination to exist between two terms both terms must exist.”16 But the agent’s action or effect does not exist in some “broad way,” since what actually comes to be cannot be a “generalized” end, but some real entity, complete down to its least unique existential content.

Again, the sufficient reason for a given end being reached cannot be merely a reason for some abstract, broadly-defined terminus ad quem. Rather, it must be a unique reason for the concrete existential conditions of what actually comes to be. Just as when one aims to graduate from college, he does not achieve this end abstractly, but rather with a concrete, unique set of courses and grades. So, too, the end “foreknown” by the intelligent director of non-knowing agents must be foreknown in its unique existential details, not merely as some “broadly conceived end.”

Among beings who do not qualify as such an intelligent director are human beings, whose knowledge of the end is limited to “broadly conceived ends” – since our inherent epistemic limits preclude perfect knowledge of anything, much less ahead of time.

Indeed, what kind of mind can possess such perfect knowledge of anything down to its least existential detail, its intrinsic metaphysical composition? And do so even before the thing effected comes into being? Such knowledge, not only exceeds the boundaries of all material technology, but, perhaps as well, that of any finite knower bound by the restrictions of temporal existence.

Does not this kind of knowledge of the actually achieved ends of all finite agents hint at the existence of an intelligent governor who transcends the limits of time and space? Could this be how the fifth way leads ultimately to a single Intelligent Governor of all finite agents, who is God?

The Fifth Way's Explicit Claims

Maritain employs an example of a chemical reaction that appears to have universal regularity. But the force of Maritain’s reasoning for the need for an intelligent director to an end applies even if no universal laws of nature exist and every conceivable agent has a unique end. Despite the fifth way’s statement about “natural bodies” acting “always … in the same way,” that claim of regularity is not essential to its argument. What is essential is the need, as Maritain puts it, for the end to be “preconceived by a thought, so as to preordain the agent to that action.”17 That is why St. Thomas points out elsewhere that there is a need to “know the nature of an end as an end.”18

The fifth way’s argument actually advances just two essential claims:

(1) “Things which lack knowledge, such as natural bodies, act for a [definite] end.”

(2) “Those things which do not have knowledge do not tend toward an end unless directed by something with knowledge and intelligence.”

Both claims have been demonstrated above, employing texts from St. Thomas as well as added arguments, such as Maritain’s. St. Thomas concludes from these premises: “There exists some intelligent being by whom all natural things are directed to an end.”

Skeptics will see mention of regularity in the behavior of natural bodies as a faulty overgeneralization from particulars, since the claim that all natural bodies “act in the same way, so as to attain that which is best” fails to be proven. Yet, St. Thomas’ mention of regularity in nature underlines the seeming universal governance by God of the whole world as known by both common sense – and also by natural science, since science necessarily operates by presuming the regularity of all cosmic phenomena.

As I have shown above, the fifth way has more fertile implications than its explicit claims appear to indicate.


  1. Contra Gentes, III, c. 3.
  2. Leonine edition, translation mine.
  3. Summa Theologiae, I, q. 44, a. 4; I-II, q. 1, a. 2, c; Contra Gentes, III, c. 2.
  4. Dennis Bonnette, Aquinas’ Proofs for God’s Existence (Martinus-Nijhoff: The Hague, 1972) 162-167.
  5. Summa Theologiae, I-II, q. 1, a. 2, c.
  6. Ibid.
  7. Ibid.
  8. Summa Theologiae, I, q. 1, a. 1, ad 1.
  9. Jacques Maritain, Preface to Metaphysics (London: Sheed and Ward, 1945) 117.
  10. Ibid.
  11. Ibid., 117-118.
  12. Ibid., 119.
  13. Summa Theologiae, I-II, q. 1, a. 2, c.
  14. Summa Theologiae, I, q. 2, a. 3, c.
  15. Summa Theologiae, I, q. 11, a. 3, c.
  16. Maritain, Preface to Metaphysics, 117-118.
  17. Ibid., 119.
  18. Summa Theologiae, I-II, q. 1, a. 2, c.
Dr. Dennis Bonnette

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Dr. Dennis Bonnette retired as a Full Professor of Philosophy in 2003 from Niagara University in Lewiston, New York. He taught philosophy there for thirty-six years and served as Chairman of the Philosophy Department from 1992 to 2002. He lives in Youngstown, New York, with his wife, Lois. They have seven adult children and twenty-five grandchildren. He received his doctorate in philosophy from the University of Notre Dame in 1970. Dr. Bonnette taught philosophy at the college level for 40 years, and is now teaching free courses at the Aquinas School of Philosophy in Lewiston, New York. He is the author of two books, Aquinas' Proofs for God's Existence (The Hague: Martinus-Nijhoff, 1972) and Origin of the Human Species (Ave Maria, FL: Sapientia Press, third edition, 2014), and many scholarly articles.

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