Inertia and the Existence of God

Aristotle, Impetus, and the Corruption of Thomism

One of the most interesting chapters in the history of theology is the debate over the arguments for the existence of God. You may be thinking that I am speaking of the debates between atheists and theists. I am not. In the history of scholastic theology, one of the most contentious points between the various schools concerned which arguments for God’s existence were valid and what degree of certitude could be drawn from each argument.

The atheistic critiques of the arguments for the existence of God look like the work of a child compared to some of the very complex and involved critiques offered by certain scholastics and the responses given by others. While many are aware of the dispute over the Ontological Argument, this is not the only argument that was debated by the scholastics. Famously, the Five Ways of St. Thomas elicited many lengthy critiques from other scholastic theologians, as well as responses from the Thomists of the strict observance.

While the article from Vini is not considered one of these classical debates over a particular argument for the existence of God, it is a classical debate between scholastics that is relevant to defending the arguments for God’s existence against contemporary attacks. In this way, it serves as an exemplar of how these scholastic disputes can be used constructively.

Ite ad Thomam,

Christian B. Wagner

Inertia and Aquinas’ First Way — Aristotle, Impetus, and the Corruption of Thomism

In his monumental work God: His Existence and His Nature, Reginald Garrigou-Lagrange presents the five Thomistic proofs of the existence of God and undertakes to defend them against some modern attacks. The first proof — the proof from motion — stands or falls with two foundational propositions. The first, and most contested, is the so-called Aristotelian kinesiological principle:

Omne autem quod movetur, ab alio movetur.

“Whatever is moved is moved by another.”

— Aristotle, Physics VII.1, 241b24; St. Thomas Aquinas, Summa theologiae I, q. 2, a. 3

For Aristotle, motion is defined as follows:

“The fulfilment of what exists potentially, in so far as it exists potentially, is motion.”

— Physics III.1, 201a10–11

But such actualization must occur in some order of being. And since being is divided according to the ten categories, Aristotle asks in the third book of the Physics in which categories motion can properly occur. After a careful analysis, he concludes that it can only be in four of them: substance, quantity, quality, and place. Substance changes by generation or corruption; quantity by augmentation or diminution; quality by alteration; and place by local motion. Hence his enumeration:

“There are six sorts of movement: generation, corruption, augmentation, diminution, alteration, and change of place.”

— Aristotle, Categories 14, 15a13–14

Now, the modern critic seizes upon one of these six: local motion, or change of place. And the attack runs as follows. Newton’s first law of motion states that a body continues in its state of rest or of uniform rectilinear motion unless acted upon by an impressed external force. No continuously acting pusher is required. The body, once set in motion, simply persists. Therefore, says the critic, the kinesiological principle is false — as, after contact with the thrower ceases, an object in linear motion is being moved, but not being moved by another. And consequently the First Way collapses.

This is perhaps the most common modern attack on the First Way.

How does Garrigou-Lagrange respond?

By attempting to bite the bullet entirely.

He grants the premise: yes, the principle must be understood as demanding a present mover for every body in motion. He writes:

“There is no less change in the transition from one position to another in the course of movement, than in the transition from repose to motion itself; if, therefore, this first change demands another cause, the following changes demand it for the same reason.”

— Garrigou-Lagrange, God: His Existence and His Nature, vol. I

Following another French Thomist, Goudin, he concludes that after a projectile leaves the hand of its thrower, an impetus is impressed in the object — a force actively moving it from within. This impetus is finite; it therefore cannot sustain infinite motion. And so, for Garrigou-Lagrange, the principle of inertia itself must be contested. There is a conjoined mover of the projectile in motion, but motion cannot be the way inertia claims it is.

The thesis of this article is that this response is flawed. It does not save Thomism. It confuses Thomism with Averroism and creates an unnecessary conflict with modern science and the principle of inertia.

It defends the First Way by adopting the very interpretation of Aristotle that St. Thomas explicitly identified and explicitly rejected in Averroes — calling it, in the words of the Angelic Doctor, omnino falsum — altogether false.

As we shall see, the genuine Thomistic position does not conflict with inertia. When the principle is rightly understood, it can be defended with far greater philosophical precision — and without the unnecessary embarrassment of contesting a well-established physical law.

My aim in this article is therefore fourfold: to distinguish the two interpretations of the kinesiological principle; to trace their historical origins; to establish the authentic Thomistic and Aristotelian doctrine by argument; and to demonstrate its full compatibility with Newtonian inertia and modern science.

This matters not only for the history of philosophy but for the apologetics of the present day. The First Way, correctly understood, is not touched by Newton. The bullet aimed at Aquinas hits only Averroes. And the Thomist should not jump in front of it.

To pursue this, I will divide the argument into five theses, followed by formal objections and replies, and a conclusion.

Thesis I: Aristotle’s authentic teaching on the kinesiological principle and on the three modes of local motion is the so-called “mitigated” view.

Thesis II: Avicenna and Averroes deviated from Aristotle and introduced the idea of motor coniunctus.

Thesis III: Latin scholasticism was contaminated by Arabic Aristotelianism, creating a false impetus theory.

Thesis IV: St. Thomas’ authentic doctrine is true to Aristotle, it is philosophically provable, it is consistent with Aristotle, and it is compatible with inertia.

Thesis V: The Thomistic school initially followed the authentic Thomistic view, but gradually fell away from it.

I. Thesis I: Aristotle’s Teaching on the Kinesiological Principle

In Aristotle’s philosophy, local motion admits a tripartition of fundamental importance for everything that follows. In Physics VIII, chapter 4, Aristotle writes:

“Of things to which the motion is essential some derive their motion from themselves, others from something else: and in some cases their motion is natural, in others violent and unnatural.”

— Aristotle, Physics VIII.4, 254b7–15

Three modes emerge: natural motion, violent or compulsory motion, and voluntary or self-motion. Let us take each in turn.

Natural Motion

Natural local motion is the motion that proceeds from the intrinsic nature of a body, as from an internal formal principle. In Aristotle’s elemental physics, the paradigm cases are the downward motion of heavy bodies and the upward motion of light ones. This motion is not imposed from without; it arises from what the thing is. This is why Aristotle defines nature itself as the principle of motion and rest in the thing to which it belongs, per se and not as a concomitant attribute (Physics II.1, 192b21–23). Nature is literally defined as the source of natural motion.

Notice immediately what this does not say. It does not say the substantial form is an efficient mover pushing the body. It says the form is the principle — the ἀρχή — from which motion flows. The word is not αἰτία, not causa efficiens. This terminological precision will become decisive when we reach St. Thomas’ correction of Averroes.

Furthermore, Aristotle is explicit that natural inanimate bodies do not move themselves in the strong sense. They have a principle of motion within them, but it is a passive principle, not an active self-causing one:

“So it is clear that in all these cases the thing does not move itself, but it contains within itself the source of motion — not of moving something or of causing motion, but of suffering it.”

— Aristotle, Physics VIII.4, 255b29–31

The stone falls downward, but the stone does not move itself. The falling proceeds from the formal nature of the stone as heavy, but the formal nature is a principium patiens — a passive principle — not a motor. Aristotle says elsewhere:

“By heavy I mean that which naturally moves toward the center; by light, that which naturally moves away from the center.”

— Aristotle, De caelo I.3, 269b23–25

The stone moves downward because it is heavy. Its heaviness is the formal source of its natural motion. But the stone is not the mover of itself. This leaves open the question: what, then, is the per se efficient cause of natural inanimate motion? Aristotle does not resolve this question explicitly in the most formal terms as he does with violent motion and voluntary motion — which we will see shortly. He gestures toward the generator, the one who produced the body’s nature. But he does not develop a crisp, formal theory of the generator as the sole per se efficient cause. Here we simply note the gap.

Violent Motion

Violent or compulsory local motion is motion imposed upon a body contrary to its natural inclination. Aristotle defines it clearly:

“whose moving principle is outside, the thing compelled contributing nothing.”

— Aristotle, Nicomachean Ethics III.1, 1110b15

And he notes its derivative character:

“An unnatural movement presupposes a natural movement which it contravenes.”

— Aristotle, De caelo III.2, 300a24–25

Violent motion is, in a precise sense, parasitic upon natural motion. We call the upward motion of a stone “violent” only because the stone naturally tends downward. Violence is defined by contrariety to nature.

Now Aristotle is aware of the obvious difficulty: a stone thrown upward continues to move after it has left the hand. The mover — the hand — is no longer in contact. Yet the motion continues. What moves it?

Aristotle’s answer, elaborated in Physics VIII and De caelo III, is the medium: the surrounding air. The original mover gives not only motion but the power of moving to the air. The air, naturally suited to move and be moved, carries the projectile onward. And Aristotle adds an important argument for why this power must be in the medium and not in the projectile:

“If the air were not endowed with this function, constrained movement would be impossible.”

— Aristotle, De caelo III.2, 301b29–30

His reasoning: if the power of moving were an internal source of the projectile itself, then violent motion would arise from an intrinsic principle — but that would make it natural, not violent. Violence is always ab alio, always from without. Therefore the motive power must reside in the medium, not the projectile.

Now, this theory of the medium is scientifically indefensible. We know — as the Christian commentator on Aristotle, John Philoponus, already argued in the sixth century — that air does not explain projectile motion. Block the air around a spinning disk and it does not stop; a heavier stone is thrown farther than a lighter one, contrary to what the medium theory would predict. Aristotle’s account of violent motion fails on empirical grounds — and the ancients already saw it.

But notice: the argument structure is not entirely wrong. What Aristotle correctly identifies is the fundamental point that violent motion cannot have its source in an intrinsic natural principle of the body. His error is in designating the medium as the extrinsic mover. The correct response is not to invent an internal impressed force — that would simply re-introduce the very intrinsic source Aristotle rightly rules out. The correct response is what the later Thomists articulate: the impressed force (impetus), if it exists, is not a motor, but an instrumental principle from the original mover. We will develop this fully in Thesis IV.

Before leaving violent motion, it is worth making a formal distinction that Aristotle does not make explicitly but which resolves his error:

Distinction: Internal as Natural vs. Internal as Impressed

Now, an important subtlety must be analyzed, so that the correct meaning of what we are saying may be rightly understood. Aristotle’s argument that the source of violent motion cannot be in the projectile conflates two senses of “internal”:

First sense: an intrinsic, natural, permanent principle flowing from the projectile’s substantial form. In this sense, we grant Aristotle: no such source of violent motion exists in the projectile. If it did, the motion would not be violent at all.

Second sense: an impressed, borrowed, accidental force received from an extrinsic mover, contrary to the body’s natural inclination, residing in it as in a subject. In this sense, there can be something in the projectile without making the motion natural or self-originated.

Aristotle’s argument succeeds against the first sense. It does not succeed against the second. And it is in this second sense that the later Thomistic tradition would speak of an impressed impetus — provided (as we shall insist) impetus is not construed as a mover but only as an instrumental principle.

Voluntary or Self-Motion

The third mode of local motion belongs to animals, which move themselves by their own parts. Aristotle is explicit:

“Self-movement is a characteristic of life and peculiar to living things.”

— Aristotle, Physics VIII.4, 255a6

The animal moves itself, but not in a simple or confused sense. The will moves the nerves; the nerves move the muscles; the muscles move the limbs; the limbs move the body locally. The efficient causality proceeds through really distinct parts. Aristotle analyzes this in a lot of detail in his treatise De Motu Animalium. An interested reader can refer to that work. But for us, for now, it suffices to note that the kinesiological principle applies here straightforwardly: one part is moved by another.

Conclusion of Thesis I

Recapitulating: local motion is threefold — natural, violent, and voluntary. For voluntary animal motion, the principle finds its mover in the moving part. For violent motion, Aristotle designates the medium, though this is empirically untenable and will require correction. For natural inanimate motion, Aristotle correctly identifies the principle of motion as the formal nature of the body — but he does not yet make fully explicit that the sole per se efficient cause of natural motion is the generator of the form. He gestures toward it, as we will demonstrate later, and he explicitly denies that inanimate bodies move themselves. The explicit development of this point belongs to St. Thomas Aquinas.

Two things are already established by Aristotle’s own text. First: the form of a heavy body is not an efficient mover. It is a passive formal principle. Second: natural inanimate bodies do not move themselves. With these two points, the Averroistic corruption is already, in principle, excluded by Aristotle himself.

II. Thesis II: Avicenna and Averroes’ Corruption

The Arabic Aristotelian tradition did not simply transmit Aristotle. It transformed him. And the transformation was nowhere more consequential than in the interpretation of the kinesiological principle.

Avicenna read Aristotle through a distinctly Neoplatonic lens. For him, matter and form are not two co-principles of a single substance, as in genuine Aristotelian hylomorphism, but two quasi-autonomous res, two things that stand over against each other. This dichotomy makes the form look like an active agent and the matter like a passive subject upon which the agent acts. From this, Avicenna concluded that the substantial form of an inanimate body is the motor coniunctus — the conjoined mover — of that body’s natural motion. The form of heaviness actively moves the matter downward, as a kind of immanent engine.

Against this, we lay down our first conclusion of this thesis:

Conclusion 1: The Substantial Form is Not the Efficient Mover of Natural Inanimate Motion

First Argument: From the Impossibility of Self-Motion in Inanimate Bodies

Major: If the substantial form of an inanimate body were the efficient cause of its natural local motion, that body would move itself.

Minor: But heavy and light bodies do not move themselves.

Conclusion: Therefore, the substantial form of a heavy or light body is not the efficient cause of its natural local motion.

Proof of the major: the substantial form is intrinsic to the body. If it were not merely the formal principle but the active efficient mover of the body’s own motion, the mover and moved would both be found within the same inanimate whole — and that is precisely what self-motion means.

Proof of the minor: Aristotle is unambiguous:

“It is impossible to say that their motion is derived from themselves: this is a characteristic of life and peculiar to living things.”

— Aristotle, Physics VIII.4, 255a5–6

“Therefore none of the things that we are now considering move themselves... in each case the movent must be separate from the moved.”

— Aristotle, Physics VIII.4, 255a13–16

Self-motion is the prerogative of the living. To attribute self-motion to a stone is to collapse the distinction between animate and inanimate — a distinction Aristotle regards as fundamental.

Second Argument: From the Distinction of Formal and Efficient Causes

Major: The formal cause as such must not be identified with the efficient cause as such.

Minor: The substantial form is the formal cause of the natural body, while the efficient cause of motion is the primary source of change, really distinct from the thing changed.

Conclusion: Therefore, the substantial form, as substantial form, is not the efficient cause of natural motion.

Proof of the major: Aristotle explicitly distinguishes the four causes, identifying the formal cause as “the form or archetype, the statement of the essence,” and the efficient cause as “the primary source of the change or coming to rest,” such as “the father,” and generally “what causes change of what is changed” (Physics II.3, 194b26–29). The two are not the same; confusing them is a fundamental category error.

Third Argument: From Aristotle’s Own Text on the Passive Principle

Major: That which is a principle of being moved — of suffering motion passively — is not thereby an efficient cause of motion.

Minor: Aristotle explicitly states that heavy and light bodies have within themselves a principle of motion, but a principle of suffering it, not of causing it.

Conclusion: Therefore, the principle in these bodies is a passive formal aptitude, not an efficient cause.

Proof of the minor. This is perhaps the most decisive Aristotelian text against Avicenna:

“So it is clear that in all these cases the thing does not move itself, but it contains within itself the source of motion — not of moving something or of causing motion, but of suffering it.”

— Aristotle, Physics VIII.4, 255b29–31

The principle the body has within itself is principium patiens, not principium agens. Not a principle of moving, but of being moved. Avicenna inverts this.

Averroes saw part of the problem with Avicenna. If the form is simply the efficient mover of the body, inanimate bodies become self-movers in too strong a sense, obliterating the distinction between living and nonliving. Averroes therefore refined the account: the form does not move the body directly, but moves the medium, and the medium then moves the body. In natural falling, the heavy body’s form acts on the surrounding air; the moved air then acts on the body; and so the body moves itself per accidens through the medium, much as a rower moves himself through the boat by rowing against the water.

This is why Averroes concludes that a resisting medium is indispensable not only for violent motion but for all natural local motion. Motion in a void, he also concludes, would be absolutely impossible.

Conclusion 2: Averroes’ Doctrine is Also False

Against Averroes, two lines of argument suffice at this stage.

First, the arguments from the confusion of formal and efficient principles still apply in full. Whether the form acts directly or through a medium, if it acts as an efficient cause, it is still a motor coniunctus, and the previous arguments stand.

Second, and decisively, the Averroistic thesis implies that motion in a vacuum is impossible. But motion in a vacuum is not impossible — indeed the whole of modern physics presupposes it. Modern science has definitively closed this question. The Averroistic framework is empirically refuted. This alone demonstrates that it cannot represent the authentic philosophical truth about motion.

St. Thomas, as we shall see, reached the same conclusion by pure philosophical analysis — six centuries before Newton.

III. Thesis III: The Contamination of Latin Scholasticism

When the Arabic commentaries entered Latin scholasticism in the twelfth and thirteenth centuries through the translations of Michael Scot and others, they brought with them this strengthened interpretation of the kinesiological principle: whatever is in motion must be being moved here and now by another, through a present motor coniunctus. This was not Aristotle’s own position, as we will prove. But it came labeled as Aristotle, and it came with the enormous authority of Averroes.

The Latin West was simultaneously being influenced by another tradition of criticism. John Philoponus, a Christian commentator of Alexandria in the sixth century, had already attacked Aristotle’s medium theory of projectile motion on empirical grounds. He noted, among other arguments, that if one surrounds a spinning disk with a container blocking the air on all sides, the disk continues to spin — demonstrating that the medium is not what sustains the motion. He concluded:

“A certain incorporeal motive power is given to the projectile through the act of throwing.”

— John Philoponus, Commentary on Aristotle’s Physics, IV.8

Philoponus did not develop this in great detail, but the intuition was there: the motive power, according to him, seemed to be in the projectile, not the medium.

Now the collision occurred. The Latin scholastics received Averroes’ strong reading of the principle — every moving thing needs a present motor — and simultaneously recognized that Aristotle’s medium theory was untenable. Something had to give.

Their solution: accept Philoponus’ intuition, but interpret it through the Averroistic lens. If every moving thing needs a present motor, and if the medium is not that motor, then the motor must be in the projectile itself — as an impressed quality, a virtus motiva, what came to be called impetus.

Francis de Marchia (a Franciscan, writing around 1320) was the first to develop this explicitly, in the context of sacramental theology. Jean Buridan, twice rector of the University of Paris, systematized it into a full theory. Buridan writes:

“The mover, in moving the mobile, impresses into it a certain impetus, a certain power able to move that mobile in the direction in which the mover moved it.”

— John Buridan, Quaestiones super octo libros Physicorum, VIII, q. 12

The crucial point is this: the impetus theorists were not abandoning the strong Averroistic interpretation of the principle. They were desperately trying to save it. Once Aristotle’s medium theory collapsed, they invented an internal impressed force so that every moving thing would still have its present motor — now located within the projectile itself rather than in the surrounding air.

The notion of a motor coniunctus for every natural motion led medieval thinkers to seek some intrinsic mover even in freely falling bodies, and the theory of impetus was an attempt to preserve the principle omne quod movetur ab alio movetur in its strong Averroistic form.

But this entire development rested upon an Averroistic/Avicennian distortion of Aristotle. Aristotle had not said that every body in motion must here and now be moved by a present conjoined mover. He had said that whatever is reduced from potency to act depends upon another already in act. The stronger interpretation was an Arabic importation. The impetus theories were the Latin attempt to preserve that importation after Aristotle’s own physics had become untenable.

The true Aristotelian and Thomistic response is different: not to invent new internal motors, but to clarify that the kinesiological principle never demanded one in the first place.

IV. Thesis IV: St. Thomas’ Authentic Doctrine

St. Thomas Aquinas commented with great care on the kinesiological principle and on the philosophy of motion throughout his career. His position is not simply a reception of Aristotle: it is an active, philosophically rigorous defense of the genuine Aristotelian sense against the Averroistic corruption, with important developments of his own.

His central text for our purposes is from his commentary on Aristotle’s De caelo. Averroes had argued that the form of heavy and light bodies acts as an active efficient principle, requiring resistance, moving the medium, and moving the body per accidens. Thomas responds:

“[Averroes] thought that the form of heavy and light bodies is an active principle of motion after the manner of a mover needing some resistance contrary to the tendency of form, and that motion is not immediately due to the agent who conferred the form. But this is absolutely false. The form of heavy and light bodies is a principle of motion not as a generator of motion but as a means by which the mover moves, just as colour, a principle of sight, is a means by which something is seen... Thus the movement of heavy and light bodies does not come from the generator by the intervention of another moving power. Nor even is there any need to look for resistance here other than that which exists between generator and generated. Thus it follows that air is not required for natural motion of necessity, as in the case of violent motion, since that which moves naturally has a force imparted to it which is a source of motion. Consequently there is no need for a body to be moved by any other force impelling it, as though it were a case of violent motion, having no implanted force from which motion springs.”

— St. Thomas, In III De Caelo, lect. 7, n. 9

Two things stand out immediately. First, Thomas calls Averroes’ view omnino falsum — altogether false. Not partially false, not nuanced — altogether. Second, he provides the precise reason: the form is not an efficient mover, but the means quo — the means by which — the mover moves. Just as colour is not a seer but the medium of seeing, so the form is not a mover but the medium of natural motion. The efficient cause remains the generator who imparted the form in the first place.

This is confirmed throughout his corpus:

“In heavy and light bodies there is a formal principle of their motion; but not in such a way that the natural form is the mover. Rather, the mover is the generator, which gives such a form, upon which such motion follows.”

— St. Thomas, In II Physicorum, lect. 1, n. 4

And with remarkable metaphorical clarity in his opuscule De motu cordis:

“Upward motion is natural to fire because it follows upon its form; and hence the generator which gives the form is the per se mover according to place. Just as a natural motion follows upon the form of an element, so nothing prevents other forms from giving rise to other natural motions.”

— St. Thomas, De motu cordis, ed. Parma, XVI, 359a

The doctrine is unambiguous, systematic, and repeated across every genre of Thomas’ writings. Let us now develop it formally.

Distinction I: Principium and Causa

A principium (principle, ἀρχή) is that from which something flows in any way whatsoever. It is a strictly relational and formal term. It is not an absolute thing in its own right but the source from which a principiatum necessarily proceeds. A formal principle does not act upon something distinct from itself as an efficient cause does; it is the immanent root from which characteristic activities emanate spontaneously and inevitably.

A causa efficiens (efficient cause) is an absolute entity, really distinct from the effect it produces, upon which the effect depends for its coming-into-being. The efficient cause exercises actual agency upon something other than itself.

Nature (physis, natura) as a substantial form is a principium — the active formal source from which characteristic properties, rest, and motions flow necessarily and immediately. It is not thereby an efficient cause of those same motions. As Thomas states in De veritate, q. 22, a. 3:

“The forms of corporeal things cannot be movers, although they can be the principle of motion, as by which something is moved; just as in the motion of the earth, gravity is the principle by which it is moved, but it is not the mover.”

Gravity is not a little engine pushing the stone. It is the formal source from which downward motion flows as naturally as heat flows from fire. The mover is the generator. The principle is the form.

Distinction II: Actus Primus and Actus Secundus

Thomas, following Aristotle, distinguishes two senses of act. Actus primus is the first actualization of pure potentiality by substantial form — the production of a new substance through generation. This requires a genuinely distinct efficient cause, a generans, in actual contact with the matter.

Actus secundus is the exercise or realization of a capacity already fully present in the first act. The geometer who already possesses the science of geometry does not need a new teacher to geometrize; he already has everything required. All that stands between him and geometrizing is the possible presence of an external impediment.

The kinesiological principle — Omne quod movetur ab alio movetur — applies directly and properly to the reduction of radical potentiality to actus primus. It governs generation, the production of new natures. Once a nature is constituted in actus primus, passage to actus secundus requires no new per se efficient cause. The nature operates statim — immediately — and per seipsum, unless there is an impediment. Thomas:

“It is not necessary that it be reduced to a second act by some agent; it operates immediately by itself, unless there is something hindering it.”

— St. Thomas, In VIII Phys., lect. 8, n. 3

With these two distinctions established, the formal arguments follow.

Conclusion I: No Motor Coniunctus is Needed for Natural Motion

Argument from the Nature of a Formal Principle

Major: That which possesses within itself an active formal principle (principium formale activum) of some motion does not require a distinct conjoined efficient mover to produce that same motion.

Minor: But natural inanimate bodies (heavy and light) possess within themselves an active formal principle of their characteristic natural motion.

Conclusion: Therefore, natural inanimate bodies do not require a distinct conjoined efficient mover (motor coniunctus) to explain or sustain their characteristic natural motion. The generans which produced the form is the per se efficient cause; once nature is constituted, it is sufficient to itself for motion.

Proof of the major: An active formal principle is precisely that a quo motion flows immediately (statim), necessarily, and spontaneously, given no impediment, without the intervention of another agent. The function of a principium is not to cause motion by acting on something distinct from itself — that belongs to efficient causality — but to be the inherent source from which motion emanates as from a root. As color is the formal principle by which sight is exercised (without color being a seer), so the substantial form is the formal principle by which motion occurs without being itself a mover. Thomas is explicit: “The forms of corporeal things cannot be movers, although they can be the principle of motion, as by which something is moved.” (De verit., q. 22, a. 3). Where a formal principle is sufficient, an efficient mover is superfluous.

Proof of the minor: Aristotle defines nature precisely as “the principle of motion and rest in those things to which it belongs per se and not as something concomitant” (Phys. II, 1, 192b21–23). In heavy and light bodies, the substantial form is the formal root from which their characteristic local motion follows necessarily, just as their other accidents follow. Thomas states: “In heavy and light bodies, however, it is the formal principle of their movement, because just as other accidents follow substantial form, so also place, and consequently being moved to place.” (In II Phys., lect. 1, n. 4). Empirical confirmation: fire always rises and earth always falls.

The conclusion: the sole per se efficient cause of natural inanimate motion is the generator who produced the form. No additional motor accompanies the falling stone.

Argument from the Two Senses of Act

Major: A body already fully constituted in its actus primus (first act) requires no additional per se efficient cause — no supervening mover — to pass to actus secundus (the exercise of its act).

Minor: But a heavy body in free fall — or any body in natural motion — is precisely a body already constituted in its first act, passing to second act.

Conclusion: Therefore, the natural motion of a heavy or light body requires no per se efficient cause beyond the original generans and (if applicable) the removens prohibens. No motor coniunctus need be invoked.

Proof of the major: Aristotle proves this by the analogy of knowledge (Phys. VIII, 4, 255b1–12): one who possesses geometry in habitu has everything needed to geometrize in actu; no new teacher is required to bring him to actual geometrizing. All that can intervene is either the removal of an external impediment or the will’s own self-determination. The one who taught him in the first place (docens) is therefore the per se cause of both the habitus and of all that necessarily follows from it. Thomas applies this exactly: “It is not necessary that it be reduced to a second act by some agent; it operates immediately by itself, unless there is something hindering it.” (In VIII Phys., lect. 8, n. 3)

Proof of the minor: The heavy body already is a heavy body — it possesses the substantial form from which downward motion follows as from a formal principle. It is not in potency to being heavy; it is heavy in first act. Its downward motion is simply the realization (actus secundus) of what it already fully is. Aristotle: “air is actually light, and will at once realize its proper activity as such unless something prevents it” (Phys. VIII, 4, 255b9–11).

Conclusion II: The Mitigated Interpretation is the Authentic Thomistic Sense

First, from explicit textual testimony. Thomas states the same doctrine explicitly, consistently, and repeatedly across every type of his writings:

• “non tamen ita quod forma naturalis sit motor, sed motor est generans, quod dat talem formam, ad quam talis motus consequitur.” — The natural form is not the motor; the motor is the generator who gives the form from which motion follows. (In II Phys., lect. 1, n. 4)

• “Sic igitur generans est per se movens gravia et levia, removens autem prohibens per accidens.” — The generator is the per se mover of heavy and light bodies; the remover of obstacles is only per accidens. (In VIII Phys., lect. 8, n. 7)

• “formae rerum corporalium non possunt esse moventes, quamvis possint esse motus principium.” — Corporeal forms cannot be movers, though they can be principles of motion. (De verit., q. 22, a. 3)

• “Formam enim ipsius elementaris corporis sequitur talis motus... generans dicitur esse movens inquantum dat formam quam consequitur motus.” — Such motion follows the form of the elementary body; the generator is called the mover insofar as it gives the form. (De pot., q. 5, a. 5)

• “impossibile est quod aliqua natura inclinet ad motum secundum seipsum.” — It is impossible that any nature incline toward motion for its own sake. (Sum. cont. gent., III, c. 23, §6)

This is not a marginal or ambiguous position. It is systematic, repeated, argued, and polemically defended against Averroes with the judgment omnino falsum.

Second, from grammatical precision. The Latin passive movetur — and its Greek original κινούμενον — refer to being moved, to the actuation of a passive principle, not to being currently in motion in the active sense. Thomas never writes omne movens ab alio movetur (every mover is moved by another) — that would contradict the unmoved First Mover — nor omne in motu ab alio movetur (everything in motion is being moved by another). The subject movetur refers to nature as passive material principle: to that which was moved in the generation of a new substance, not to that which is currently in the realization of its second act.

Third, from the structure of the First Way itself. St. Thomas’ First Way in Summa theologiae I, q. 2, a. 3 is not an argument about continuous motion requiring continuous pushing. Even though many Thomists tried to see it that way, it isn’t — this unfortunate reading can be seen not only in Lagrange, God: His Existence and His Nature, but also in Billuart, Summa S. Thomae, I, De Deo et ejus attributis, diss. I, art. II; Gonet, Clypeus I, De Deo uno, disp. I, art. II; Goudin, Philosophia Thomistica, II, Physica, pars I, disp. III, q. VI, arts. I–II; Hugon, Tractatus dogmatici, vol. I, De Deo uno, q. I, art. II; and many others. And nothing in the text indicates such a view. It is an argument about the reduction of potency to act: “Nothing can be reduced from potency to act except by something in act.” That is the metaphysical center. The principle as St. Thomas deploys it in the First Way concerns the actuation of real potencies — primarily the production of new natures — not the perpetuation of motion already in actus secundus.

Conclusion III: The Mitigated Interpretation Is Also Aristotle’s Own Authentic Sense

This must be demonstrated explicitly, because the strong reading is habitually presented as “Aristotle’s view” — particularly by modern historians of science, from Duhem to Dijksterhuis. Weisheipl states the correction bluntly: “the position attributed by modern historians to Aristotle is, in fact, the position of Averroes.” And: “the notion of a substantial form of inanimate bodies being a motor coniunctus or causa efficiens motus is entirely foreign to Aristotle’s own philosophy.” Furthermore: “the Greek equivalent of the phrase motor coniunctus is nowhere found in Aristotle.”

Three arguments establish this conclusion.

First, from the grammar of the axiom itself. The Greek original — ἅπαν τὸ κινούμενον ὑπό τινος ἀνάγκη κινεῖσθαι — employs the middle-passive participle κινούμενον, which refers to being in the state of a passive, receptive principle being actuated, not to being currently in active motion. The strong reading illicitly converts this passive into an active, transforming “is being moved” into “is moving” or “is in motion.” This is bad grammar before it is bad philosophy. The grammatical error generates the philosophical error.

Second, from Aristotle’s own words in Physics VIII, 4. Aristotle establishes the kinesiological principle by induction from the three classes of motion. Then he says:

“light things and heavy things, which are moved either by that which brought the thing into existence as such and made it light and heavy, or by that which released what was hindering and preventing it”

— Physics VIII, 4, 255b31

Anneliese Maier has correctly summarized: for Aristotle, there are only two possible efficient causes of natural motion in inanimate bodies — the generans as per se cause, and the removens prohibens as per accidens cause. No third cause — no motor coniunctus — is introduced by Aristotle.

Third, from Aristotle’s definition of nature as ἀρχή. Aristotle defines nature as the ἀρχή — the principle — of motion, not as the αἰτία — the efficient cause. Thomas notes this precisely: “principium” is placed in the definition as a quasi genus because “the word ‘nature’ implies a relationship of principle.” If Aristotle had meant nature to be an efficient mover, he would have defined it as τὸ κινοῦν, not as ἀρχὴ κινήσεως. The word Aristotle chose is itself the refutation of the strong reading.

We may therefore state: the Averroist interpretation did not have to wait until the seventeenth century to be discredited. It was already discredited in the thirteenth on strictly Aristotelian grounds by St. Thomas Aquinas.

Conclusion IV: Impetus Is Not an Efficient Motor of the Projectile

We turn now to violent projectile motion specifically. The question is not whether something like an impressed force (impetus) exists in the projectile — that is a separate and legitimate question. The question is whether impetus, if it exists, functions as an internal efficient mover, a motor, propelling the projectile from within.

St. Thomas gives the decisive text on this in the same passage of In III De Caelo:

“It is not to be understood that the power of a violent mover imparts to a stone that is moved by violence some power by which it is moved, just as the power of a generator imparts to the generated the form that results from natural movement: for then violent motion would be from an intrinsic principle, which is contrary to the nature of violent motion.”

— St. Thomas, In III De Caelo, lect. 7, n. 6

This is the mirror text to the doctrine on natural motion. For natural motion: the generator imparts the form; the form is the principle (not the motor). For violent motion: the projector does NOT impart a power that functions as the principle of motion; because if it did, violent motion would have an intrinsic source — contradicting its very definition. The two doctrines are structurally parallel and mutually reinforcing.

Argument from the Definition of Violent Motion

Major: If impetus functioned as a motor (motor coniunctus) of the projectile, then the projectile’s continued motion would arise from an intrinsic principle — namely, the impressed impetus residing in it.

Minor: But by the definition of violent motion, no intrinsic virtue (virtus indita) can be impressed into the projectile from which its continued motion arises as from an internal motive principle.

Conclusion: Therefore, impetus — under the description of an internal motor — is incompatible with the very definition of violent motion. As Thomas states: the projectile that moves violently “nullam virtutem inditam habet, ad quam sequatur talis motus” (In III De caelo, lect. 7, n. 9). The motion is always ab alio, always from without.

The major is easily seen to be true. This is precisely what the impetus theory in its strongest (Buridanian) form asserts: that the projector impresses a quality (impetus) into the projectile, and this quality thereafter acts as the proximate causa movens propelling the projectile. Buridan: “ille impetus est una qualitas innata movere corpus, cui impressa est” — the impetus is a quality capable of moving the body in which it is impressed. Under this description, the projectile would be moved from within, by its own internal quality, which is its motor.

The strength of the argument lies in the minor. Aristotle defines violent motion as “that whose moving principle is outside, the thing compelled contributing nothing” (Phys. VIII, 4, 255a2; De caelo III, 2, 301b17–18). St. Thomas explains the force of this definition: “It is not to be understood that the power of a violent mover imparts to a stone that is moved by violence some power by which it is moved, just as the power of a generator imparts to the generated the form that results from natural movement: for then violent movement would be from an intrinsic principle, which is contrary to the nature of violent movement.” (In III De caelo, lect. 7, n. 6). If an intrinsic virtue were impressed by the projector and the motion arose from that virtue, violent motion would have an intrinsic source — which is precisely what makes it not violent. Therefore the definition of violent motion logically excludes any internal motor. To deny this would be to equate violent motion to another type of motion, such as voluntary motion, which has its principle from within, or a motion in quality, which would only beg the question as to the efficient cause of local motion.

Argument from the Analogy Between Nature and Impetus (from Domingo de Soto, Sup. Lib. Phys., VIII, q. 3)

Major: Just as, in natural motion, the efficient cause (causa efficiens) is the generans and not the natural form or gravity — which are merely the principium — so too, in violent motion, the efficient cause is the proiciens (projector) and not the impetus — which is, at most, a principium or instrumental virtue.

Minor: But impetus, even conceding its existence as an accidental quality in the projectile, is an implanted, alien, and extrinsic quality — not an independent agent really distinct from and superior to the projectile.

Conclusion: Therefore, impetus — if it exists — stands to violent motion as natural form stands to natural motion: as a principium, not as a motor. The one and only per se efficient cause of the projectile’s motion is and remains the proiciens, even after the projectile has left the thrower’s hand.

The major we prove as follows. The analogy between nature and impetus is structurally precise. In natural motion: the generans gives the form, then the form is the principium (not the motor), then the form is not really distinct from the body, therefore no efficient causality inheres in the form, so the generans remains the per se cause. Domingo de Soto makes this analogy explicit: “Just as the one who generates a heavy object gives it a natural quality, which is gravity, by which it moves it all the way to the center, so too the one who throws it strikes the object by which it moves it at a distance.” (Super libros Physicorum, VIII, q. 3, fol. 104v). In both cases, the implanted quality (gravity / impetus) is the principium, not the causa efficiens. Capreolus also claims impetus can be no more than a “detached instrumental cause” of the thrower, not a mover.

The minor is easily verifiable. All impetus theorists agree that impetus is a qualitas impressa — a quality received from without, alien to the projectile’s nature, violently imposed and destined to be overcome by the projectile’s natural inclinations. It is not a substance; it is not an independent agent; it is an accident inherent in the projectile. As such, it lacks the ontological standing of a proper efficient cause (causa efficiens), which must be really distinct from, and able to act upon, the thing it moves.

Argument from the Real Distinction Required Between Mover and Moved

Major: An efficient mover (motor efficiens) of a body must be really distinct from the body it moves, such that the motion genuinely proceeds ab alio and not a seipso.

Minor: Impetus, as a quality inhering in the projectile, is not really distinct from the projectile in the required sense; it inheres in the substance as an accident in a subject, forming with it a single per accidens composite — not two really distinct agents.

Conclusion: Therefore, impetus, lacking real distinction from the subject in which it inheres and lacking suppositum, cannot exercise the role of an efficient mover (motor) upon the projectile. It can only be the vehicle — the instrumental principium — through which the original proiciens continues, in a diminishing way, to be the sole per se efficient cause of the projectile’s violent motion.

This follows from the demonstration of Phys. VII, 1: “everything that is moved must be moved by something else” (242a14–16), because to move and to be moved are really distinct acts requiring distinct subjects. St. Thomas: “To move and to be moved differ, because to move implies active causality, while to be moved implies passive causality.” (In VII Phys., lect. 1). The mover must be aliud — genuinely other — with respect to what is moved.

The minor we prove as follows: quality and its composite do not constitute two really distinct agents capable of mutual efficient causality. The quality does not act on the composite as if from outside; it is constituted in it. St. Thomas and Domingo de Soto make this clear: “Because impulse is not a suppositum, it does not act, but is the power of an agent, for example a motor.” (de Soto, op. cit., fol. 104v–105r). What lacks suppositum — subsistence in its own right — cannot properly be a mover.

Synthesis: Compatibility with Newtonian Inertia

We are now in a position to see why Newtonian inertia poses no threat to the authentic Thomistic and Aristotelian doctrine.

Newton’s first law states that a body continues in its state of rest or of uniform rectilinear motion unless compelled to change that state by impressed forces. No continuously acting pusher is required to maintain constant velocity.

The Averroistic doctrine says that every body in motion requires a present motor here and now. Therefore it is in direct conflict with Newtonian inertia.

But the authentic Thomistic doctrine says nothing of the kind. It says:

• The generator who imparted the substantial form is the per se efficient cause of natural motions.

• The projector who imparted an initial impulse is the per se efficient cause of violent motions.

• Thomists do not read the principle as “whatever is in motion is being moved by another.”

• The form and impetus are principles, not motors.

• No conjoined motor accompanies the body in natural or projectile motion.

Far from contradicting inertia, the Thomistic position structurally anticipates its key insight: a body in natural or rectilinear inertial motion does not need a continuously acting external force because it already has, from its generator, everything it needs to move.

Edmund Whittaker, a well-known physicist and historian, published a critique of the First Way arguing that the kinesiological principle “is irreconcilable with sound dynamical science.” But the irony is that he has in fact criticized the Averroist and late Averroist view on sound Thomistic grounds. Whittaker attacked Averroes. He missed St. Thomas entirely.

And interestingly, the Averroist interpretation, presented by modern historians of science as the “Aristotelian view,” did not have to wait until the seventeenth century to be discredited; it was already discredited in the thirteenth on strictly Aristotelian grounds.

V. Thesis V: The Thomistic School, the Moderns, and Garrigou-Lagrange

The authentic Thomistic tradition initially preserved the correct understanding with remarkable clarity. Capreolus and Domingo de Soto accepted the reality of an impressed force in the projectile while insisting, with equal firmness, that this force is not a proper efficient motor.

“The true efficient cause of projectile motion is the thrower. Impetus is at most a separated instrumental cause of the thrower.”

— John Capreolus, Defensiones theologiae divi Thomae, II Sent., dist. 6

Domingo de Soto makes the structural parallel explicit: the generator gives nature from which natural motion follows; the projector gives impetus from which violent motion follows. But nature is not an efficient cause; impetus is not an efficient cause. Nature is an intrinsic formal principle; impetus is an alien, borrowed, impressed principle. In both cases, the efficient cause is the one who gave the principle, not the principle itself.

The stone is thrown by the boy. It is not thrown by a little motor called impetus. The impetus is the instrumental trace of the original thrower’s causality.

Over time, however, this careful balance eroded. The scholastic manuals of the seventeenth and eighteenth centuries, increasingly influenced by a revived Averroism that came packaged as orthodox Aristotelianism, began treating the impetus as a genuine motor coniunctus. The strong reading re-entered scholastic physics through the back door.

Antoine Goudin, whose manual was widely used, exemplifies this tendency. He says in his Philosophia that the thrower imparts an impetus “capable of serving as motor.” And Garrigou-Lagrange, drawing on Goudin, makes it explicit: the impetus must be posited as a real force capable of serving as present motor. Inertia must therefore be contested.

But as Garrigou writes: “There is no less change in the transition from one position to another in the course of movement, than in the transition from repose to motion itself; if, therefore, this first change demands another cause, the following changes demand it for the same reason.” (God: His Existence and His Nature, vol. I)

This is the Averroistic structure exactly:

• Motion as presently existing requires a present motor.

• If no external motor appears, posit an internal dynamic force.

• Inertia must therefore be rejected.

The Thomistic structure is categorically different:

• Potency requires act for its actualization.

• The natural form is principium quo, not efficient motor.

• The generator gives the form.

• Motion follows upon the form as actus secundus.

• Therefore inertia is not the enemy of the First Way.

To be fair to Garrigou-Lagrange: he is not an Averroist in general. His overall Thomistic metaphysics is acceptable. But on this specific question — the correct interpretation of the kinesiological principle as applied to local motion — his defense of the First Way drifts into the very Averroistic error that Thomas explicitly repudiated.

Summary of Objections and Replies

Objection I

Aquinas says plainly: whatever is moved is moved by another. Therefore, if a stone is falling now, it must be moved by another now.

Reply

I distinguish.

If “moved” (movetur) means: is being reduced from radical potency to actus primus — I grant: it depends upon another, namely the generator.

If “moved” means: possesses continued local motion following upon a form already received and fully constituted in actus primus — I deny that this requires a conjoined external pusher at every instant.

The stone’s form is from another. Its being is from another. Its natural inclination is from another. Its motion follows upon that received nature as actus secundus. So if a stone is falling, the efficient mover of it is the one who imparted the form — for example, the air that cooled the lava, which, in turn, was being moved by the Earth, and so on. The principle is fully preserved without positing a present motor coniunctus.

Objection II

If the body continues in motion without a present mover, then motion exists without a cause.

Reply

The cause is not absent simply because it is not a present mechanical pusher. In natural motion, the cause is the generator who gave the form; in projectile motion, the cause is the projector who gave the impressed principle. In both cases, the motion depends upon another. What is denied is only the need for a present conjoined motor continuously pushing at each instant.

Objection III

Newtonian inertia says motion continues without force. Does it not deny causality?

Reply

Newtonian inertia denies the need for impressed force to maintain uniform rectilinear motion. It belongs to mathematical physics and concerns the quantitative conditions of velocity change. It does not deny the need for a cause of being, of nature, of form, of the initial conditions of motion, or of the order of actualization as such. Those questions pertain more properly to philosophy. It does not say that potency actualizes itself. It says only that, within mathematical mechanics, velocity once attained persists without continuous driving force. That is a theorem of mechanics, not a refutation of metaphysics — one that is, in turn, fully compatible with Thomism.

Objection IV

If Aquinas allows motion in a void without resistance, does he not abandon Aristotle?

Reply

No. Thomas distinguishes Aristotle’s dialectical arguments against the atomists — which were never meant as strict demonstrations of the impossibility of motion in a void — from the properly philosophical account of motion. He follows the philosopher Avempace against Averroes on this point: velocity is determined not by resistance from the medium but by the time required to cover a given distance. Natural bodies require time because they have a given distance to traverse in a quantitative space, not because they need something to push against. This is not a departure from Aristotle; it is a purification of Aristotle from Averroes.

Objection V

But if inertia is true, then local motion as uniform rectilinear motion requires no actualization at all. Does this not contradict the act-potency framework?

Reply

Not at all. Local motion, even when uniform, involves the successive actuation of real positional potencies: the body is successively here, then there. Each of these positions is a genuine actus secundus of the nature in question. What inertia denies is not that these actualizations occur, but that they require a continuously impressed external force to continue occurring. This is entirely consistent with the Thomistic account: the generator who imparted the nature is the per se cause of all that follows from it in second act, unless impeded.

Objection VI

Does not this interpretation make the kinesiological principle trivially true?

Reply

The principle as applied in the First Way is not restricted to local motion. Thomas explicitly intends movetur in the widest sense of any coming-into-being, including every actualization of potency and ultimately the actualization of esse itself. The First Way ascends from physical motion to the metaphysical order of act and potency, and from there to the conclusion that there must be a First Actuality, pure act, not reducible from any prior potency. The scope of the principle, properly understood, is fully sufficient for this ascent. Newton does not touch it.

Conclusion

We can now return to the beginning.

Aquinas opens the First Way:

“It is certain, and evident to our senses, that in the world some things are in motion.”

— St. Thomas Aquinas, Summa theologiae I, q. 2, a. 3

Then:

“Whatever is moved is moved by another.”

— St. Thomas Aquinas, Summa theologiae I, q. 2, a. 3

And the explanation:

“Nothing can be reduced from potency to act except by something in act.”

— St. Thomas Aquinas, Summa theologiae I, q. 2, a. 3

That final line in this section of the argument is the center of gravity of the First Way: not “velocity needs a pusher” but “potency cannot actualize itself.”

This is a metaphysical principle. Not a physical one.

The puck sliding on frictionless ice does not refute it.

The arrow flying after leaving the bow does not refute it.

The stone falling without a visible conjoined motor does not refute it.

None of these things has actualized itself. Each of them presupposes being, a determinate nature, an initial actualization, a causal order. Each of them is what it is because something else made it to be what it is. The principle is not about the persistence of velocity; it is about the dependency of the finite on the actual.

So the next time someone says that Newton refuted Aquinas, ask them a simple question:

Which Aquinas?

If by Aquinas you mean a caricature — the figure who believed that every body with velocity must be continuously pushed by another body — then yes, inertia refutes that. But that figure is Averroes, not Aquinas.

If by Aquinas you mean the philosopher who taught that whatever is reduced from potency to act is reduced by another already in act; that no nature generates itself; that the order of actualization is intelligible only if there is a first actualizer who is not himself reduced from potency; then inertia does not refute him. It does not even address him. It operates on a different plane entirely.

The great irony of the history of this debate is this: many of those who attacked Aquinas were in fact attacking Averroes. And many of those who defended Aquinas were in fact defending Averroes. Both sides accepted the Averroistic framing of the principle, and then fought about whether to affirm or deny it. Neither side looked carefully at what St. Thomas actually wrote.

What St. Thomas wrote, in his commentary on De caelo, is that Averroes’ view is omnino falsum. Altogether false.

The stone falling downward is not being pushed by a hidden motor. Its form is not a little efficient engine. The air is not dragging it. The generator gave the form. The form is the principle by which the motion occurs, not the motor which causes it. The motion follows upon the form as actus secundus follows upon actus primus — immediately, spontaneously, without a supervening agent.

The projectile flying through the air does not need a tiny engine inside it. If there is an impetus, it is not a supposit acting as motor. It is an instrumental virtue from the original projector — an instrument that carries, diminishingly, the causality of the one who threw.

The principle of inertia does not destroy the First Way.

It destroys only the thesis that motion, as velocity, must be continuously sustained by a conjoined pusher.

That thesis was Averroes’.

Thomas rejected it in the thirteenth century.

The true principle is deeper, and untouchable:

Whatever is in potency cannot actualize itself as potency. Whatever is moved — whatever is brought from potency to act — is moved by another. And unless there is a first unmoved mover, a First Act, a being whose actuality owes nothing to any prior potency, nothing in the universe is ultimately intelligible.

The modern critic aimed at Aquinas.

The bullet hit Averroes.

And the Thomists should stop jumping in front of it.

Appendix A: The Proofs of Omne Quod Movetur Ab Alio Movetur

The principle omne quod movetur ab alio movetur is not properly proved as though it were a remote conclusion. Rather, once the terms are correctly understood, it becomes evident. To be moved is to be reduced from potency to act; and what is in potency, precisely as such, cannot reduce itself to act. Still, because one can easily confuse motion, self-motion, nature, and causality, we can explain and manifest the principle in three ways, as does St. Thomas in the Summa Contra Gentiles.

Proof I: From the Impossibility of Primary Self-Motion in a Divisible Thing

If something moves itself, it must be moved by itself primarily and not merely through one of its parts. Otherwise, it would not be the thing that moves itself, but one part moving another, as in animals: the animal moves because one part is mover and another is moved.

Moreover, everything that is moved is divisible — as is evident from the necessity of partition into potency and act, and from the intermediary states of motion manifesting partly the terms a quo and ad quem during the motion (cf. Aristotle, Physics VI, c. 4, 234b10 ss.; St. Thomas, In Physic., lib. VI, lect. 5, n. 796).

Now, if, while one part rests, another part still moves, then the whole would not be primarily moved; rather, the part that remains in motion would be primarily moved. Therefore, if one part of it comes to rest, the whole must come to rest. But nothing that comes to rest because another comes to rest moves itself. For if the rest of one thing follows upon the rest of another, then the motion of the one also follows upon the motion of the other. Therefore, such a thing is not moved by itself.

Hence, what was supposed to move itself does not in fact move itself. Therefore, whatever is moved is moved by another.

Syllogistically:

Major. Whatever moves itself primarily must not depend, in its being moved, upon another.

Minor. But every moved thing is divisible and depends, in its being moved, upon its parts.

Conclusion. Therefore no divisible moved thing moves itself primarily; whatever is moved is moved by another.

Proof II: From Induction Through All Types of Motion

The second argument is by induction on all the types of motion.

First, as we have seen, objects are moved either per se or accidentally. But to be moved accidentally is not to move oneself, for it is to be moved by another.

Now, if the object is moved per se, it can be moved naturally, by violence, or voluntarily, as we have seen. If naturally, it is moved by the generator and by that which removed the obstacles; if voluntarily, it is the case of animals (one part moved by another); if by violence, it is evident that something is required to contradict nature.

Therefore, we can find a mover for all kinds of motion.

In syllogistic form:

Major. Everything moved falls under one of these modes: accidental motion, violent motion, natural motion of self-movers, or natural motion of heavy and light bodies.

Minor. In each of these modes, we can determine a mover.

Conclusion. Therefore whatever is moved is moved by another.

Note 1: I can see some less attentive readers might think there is some sort of circularity here — in the article we found the movers given the principle, now we prove the principle given the movers. But this is not the case. In the article, we did not assume the principle; we simply found the movers in each case of motion — and thus the principle follows.

Note 2: Throughout the article we have investigated terms such as “violent motion” and “natural motion” for local motion only, but in this case do not forget that it applies to all kinds of “violent motion,” such as the heating of water.

Proof III: From Act and Potency

This third argument is the most famous.

Nothing is, at the same time and under the same aspect, in act and in potency with respect to the same thing. But whatever is moved, insofar as it is moved, is in potency; for motion is the act of what exists in potency insofar as it is in potency. And whatever moves, insofar as it moves, is in act; for nothing acts except insofar as it is in act.

Therefore, nothing is, with respect to the same motion, both mover and moved. Therefore, nothing moves itself. Hence, whatever is moved is moved by another.

In syllogistic form:

Major. Nothing can be simultaneously in act and in potency with respect to the same thing and under the same aspect.

Minor. But the moved, as moved, is in potency; and the mover, as mover, is in act.

Conclusion. Therefore nothing is mover and moved with respect to the same motion; whatever is moved is moved by another.

This is the deepest reason for the principle. The potential, precisely as potential, does not actualize itself. What is moved must therefore be reduced from potency to act by another.

Appendix B: The Fundamentals for Such an Error

While this Appendix deserves a dedicated article, it is nonetheless interesting to outline the following thesis, given its profound structural correlation with the preceding arguments.

The great 20th-century Thomist Cornelio Fabro performed an invaluable service by identifying the various “inflections” or deviations that have historically distorted the Thomistic tradition. A primary concern of his was the introduction of the Principle of Sufficient Reason (PSR) — the postulate that every effect must possess a sufficient reason to determine why it is such and not otherwise — into the metaphysical deduction of the principle of causality. This conceptual move serves only to obscure the authentic triumph of St. Thomas: the synthesis of Aristotelian horizontal causality with Platonic vertical causality through the profound doctrine of participation.

Though frequently termed Leibnizian, the PSR was formally anticipated by the Jesuit Sylvester Maurus and finds its earlier roots in the Avicennian De Sufficientia. This principle permeated Scholastic manuals with remarkable speed, eventually contaminating even those claiming the Thomistic mantle; its influence is discernible in Gonet, Goudin, and Billuart, reaching explicit articulation in the works of Pesch, Liberatore, Zigliara, Sanseverino, Garrigou-Lagrange, and many, many others. By founding the principle of causality upon the PSR, these thinkers created a philosophical exigency to seek a motor coniunctus, hic et nunc, to account for the persistence of projectile motion. They demanded a sufficient reason at every instant for the object’s displacement, thereby making the Averroistic interpretation of the kinesiological principle necessary and the adoption of the Buridanian theory of impetus extremely suitable.

The fundamental error, as Fabro rightly insists, is that St. Thomas never employs the PSR as the ground for the principle of causality. And I would go even further than him. The proto-understanding of the PSR by Avicenna was repudiated by St. Thomas (ST Ia, q. 115, a. 6) and further rejected by Cajetan in his commentary — but this will be further expanded on another occasion.