First Way

The First Way – The Ultimate Source of Motion

Aquinas begins showing that God’s existence can be proved by reason (apart from Scripture) by offering what he considers the most obvious argument:

The first and more manifest way is the argument from motion.

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

Aquinas begins with the simple and straightforward observation that some things are moving.[1]  For Aquinas, though, ‘motion’ means more than just a change of location.  His example of fire heating wood indicates the sort of motion or change he has in mind. It is the successive replacement of one accident or characteristic of a thing by another, contrary one. Motion includes the change of any characteristics of a thing: being heated or cooled, changing size or shape or color. 

Next, Aquinas explains how such motion or change comes about.

II. Now whatever is in motion is put in motion by another,

Again, Aquinas seems to be stating something obvious, but he is making two important points about every process of change:

  1. each one has a cause,
  2. the cause is something other than what is changing.

In short, every motion is caused by something other than what is in motion; thus, as he will conclude later, nothing causes its own process of change.  He thinks, though, these points need to be demonstrated, which he does in terms of Aristotle’s notions of potency and act.  Aquinas explains that a changing thing is in the process of having its potency actualized, and it is being actualized by an external cause, which is itself actual.

Aquinas continues:

a. for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act.

  1. For motion is nothing else than the reduction of something from potentiality to actuality.
  2. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality.
  3. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it.

Aquinas is here making a logical point about what it means for something to move or change.  Motion or change is a process of acquiring a new characteristic.  In order to acquire a new characteristic, it must first have the capacity or potency for it, and second, it cannot have it already.  A changing thing at once both is not actually what it will be, and yet has the potency to become other than it is.

For wood to catch fire, it has to be capable of burning, and not yet burning.  The change or motion of catching fire is the reduction of the wood’s potential to burn to the actuality of it burning – passing from being potentially on fire to being actually on fire.  Aquinas’s main point, though, is that a thing’s ability to move or change is grounded precisely in the fact that it is not actually what it will be, but only potentially so.  Wood catches fire only as potentially aflame – actually burning wood does not catch fire or undergo the motion of ignition.  (When parts of the wood catch fire from other parts, those parts catch fire only to the extent that they are not already actually on fire, but have the potential to do so.)

For Aquinas, this means that the cause of any change must be something other than what is changing, yet the cause is active as long as the change is happening.

b. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect,

  1. but only in different respects.
  2. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold.

c. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself.

d. Therefore, whatever is in motion must be put in motion by another.

The change that a thing undergoes cannot be the cause of that very same process of change.  While the change results in a new actuality, that actuality is a result in what is first potential to it, and so the resulting actuality cannot be the actuality that brings itself about.  Wood that is actually on fire does not cause itself to become ignited – a burning match does not set itself on fire.  Anything that is changing is being made to change by some other actuating cause.

            At this point, Aquinas has shown that a thing in motion (or a process of change) is not the cause of its own motion (changing).  The cause is other than what changes.  The cause is an actuality prior to the change which is the effect.  He does not require, though, that the prior actuating cause be a motion of the same sort, or a motion at all, as some mistakenly think.[2]  For instance, the cause of heat may be hot, but it may also have some other actuality that brings about heat as an effect.  Sometimes the prior actuality is itself another motion or process of change – and this sometimes is a change of the same kind (fire igniting wood) and sometimes a motion of a different kind (friction igniting wood).

            In this First Way, Thomas is seeking to demonstrate the existence of a First Cause of Motion and, as we said above, the proper effect he has in mind is motion or physical change broadly understood (what is obvious to the senses). Thomists have historically labeled this kind of change “transient alterations:” the successive replacement of one accident by another. This would include 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, etc. succeeds another successively (through a certain duration of time) or substantial changes of generation and corruption.

            These kinds of alterations are distinguished from “immanent activities” or actuations or as Aquinas says “operations” (ST Ia 18, 3 ad 1; ST Ia, 78, 3; In DA 3, 6, no. 766) whereby cognitive faculties (sense and reason) and appetitive powers (sense desire and will) become actual, though not by one accident succeeding another over time, but all at once. 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) (I reference this distinction in the essays “Cognition: Identity/Conformity” and “Cognition in General” where the focus is not on its causality, but formal identity.)

            So, sometimes the prior actuality prior to a motion is not a motion or process of change at all, but an actuality of a different sort.  This occurs in living things, especially animals,[3] which is why Aquinas, following Aristotle, calls things which cause their own motion in this way ‘self-movers’.  By this he does not mean that their motion is causing itself, but that one part of the living thing is relatively motionless while moving the other parts.  Ultimately, it is the soul that is the unmoving cause of the motion of living things.  Yet, its activity is nevertheless caused.  This kind of causal activity is the subject of Aquinas’s Second Way.

            In this First Way, Aquinas continues by considering only the case when the prior actuality causing motion is itself a motion or a process of change.

III. If that by which it 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.

            Now, even when the prior actuality is itself a process of change, i.e. a motion, this may be prior in time, for example, when a moving billiard ball causes another on to move.  But in this First Way, Aquinas is not simply tracing motion back temporally to an initial motion from which all others followed in sequence.  For one thing, although Aquinas did believe that the universe had a first moment of creation, he is clear that he knows this only through his Christian faith.[4] He did not think it could be demonstrated from philosophy or science that the present state of the world followed from a temporally first motion.  Nor did he think that it could be logically ruled out that the physical universe always existed.  In fact, he models this argument after Aristotle who himself argued that the world was eternal yet, nevertheless, has an ultimate cause of its eternal motion.[5]  Aquinas’s philosophical proof for a first mover is not an argument for a temporal beginning but applies whether the world has a first moment or not.  So, although, in fact, he did think it was possible (though actually not the case) that the world had existed for an infinitely long time, he nevertheless believes it would still require a cause (or series of simultaneous causes) for its eternal succession of motions.[6]  The ultimate cause of motion which Aquinas is seeking to demonstrate is not the cause of the first beginning in time.

            Secondly, from his example of a hand moving a stick we know that the causal series he has in mind is not a temporal succession.  Rather, he is looking at moving causes which act simultaneously with the process of change they bring about.  He calls these causes which act simultaneously “per se causes” and says their effects are essentially subordinated, not temporally successive.[7] Other examples of essentially subordinated series of causes would include a series of interlocking cogs or a train of cars: any system of simultaneous transfer of motion with net energy output.

            Furthermore, Professor Christopher Martin[8] shows that Aquinas seeks to explain any given motion on earth through what Martin (following Peter Geach[9]) calls a ‘lumping together’ of all the motions of the world.[10]  Aquinas treats every motion of the world as part of the motion of the whole world, and through this lumping together, the motion of the whole world, and therefore all of the motions within the world, depend on a single universal moving cause.

            Aquinas accepted the astronomical and cosmological model of the physical universe that was current in his day, i.e., the model which Aristotle adopted from Eudoxus and which he describes in Metaphysics XII, 8.[11]  This geocentric model saw the earth as the motionless center of the physical universe with the celestial lights – the moon, sun, planets and stars – affixed to great transparent spheres which ceaselessly rotate around the earth in uniform circular motion.  This complex scheme explained and predicted the apparent rising and setting of the sun and moon, the variable motion of the planets and the eternal cycle of the stars.

            With this model of the physical universe, Aquinas viewed the whole cosmos as a system of essentially subordinated causes being driven by the motion of the spheres. As he says in his Summa contra Gentiles, III, 82, “The heaven must be the cause of all the movement in the lower bodies”.[12]  He also cites Aristotle in claiming that “Man and the sun generate man.”[13]  In another proof for God’s existence paralleling the First Way he says, “Everything which moved is moved by another, for lower things are moved by higher ones, as elements are moved by heavenly bodies, and these lower ones are acted upon by the higher”.[14]   In the next chapter of the same work he says, “We see, for instance, that alterations and generating and corrupting which occur among lower things are explained by the heavenly body as by a first mover, which is not moved by this same kind of movement, as it is ungenerable, incorruptible and unalterable.”[15]  It is implicit in the First Way and explicit elsewhere in Aquinas’s writings, that the heavenly spheres are per se causes of motion of the whole world, causes which act simultaneously with the motions or changes they bring about. Thus, by lumping together all the motions of the world as constituting the motion of the whole world, Aquinas believes each motion is essentially subordinated to, and simultaneously caused by, the motion of the heavens.  Continuing from the First Way:

IV. But (causes of  motion being themselves in motion) cannot go on to infinity,

  1. because then there would be no first mover, and, consequently, no other mover;
  2. seeing that subsequent movers move only inasmuch as they are put in motion by the first mover;
  3. as the staff moves only because it is put in motion by the hand.

            Now, at this point Aquinas is not describing the motion of the whole universe, but the general properties of any series of per se causes of motion. (His mention of a first mover, then, is not a reference to God.  If it were, Aquinas would indeed be engaged in a circular argument.)  Indeed, he is talking about any system of effects essentially subordinated to simultaneously acting causes.  Whenever we know that effects are the result of the simultaneous motion of prior causes which are themselves effects of prior moving causes, there must be something driving the motion of the system.  For example, a series of cogs or train cars which is transferring motion cannot be infinitely long, for there would be no motion or energy to transfer without a first cause and source for the motion supposedly being transferred.  “The point here is that in an essentially subordinated series, the only cause that is really moving anything is the first cause.  The others are like lifeless instruments.”[16] So Aquinas is arguing that (a) if the universe is an essentially subordinated system of causes, and (b) if no essentially subordinated series of causes can be infinitely long, then (c) there cannot be an infinite number of spheres driving the ultimate motion of the earth.  Instead, he concludes:

V. Therefore it is necessary to arrive at a first mover, put in motion by no other;

            It should be noted that while the first mover is actual, it is not so with the actuality of motion or change since everything in motion would require a cause for that change.  Thus, a moving ‘first cause’ could not be first.  From the fact that it is in motion, it would itself require a mover.  Moreover, if all physical beings are subject to motion and change, the first mover is not changing, and so is not physical, but nevertheless active with a non-physical actuality.


            We can now summarize Aquinas’s argument in the First Way:

  1. Things are changing/moving.
  2. Every change is caused by the (simultaneous) activity of something else.
    1. (Nothing is the cause of its own process of change.)
    2. (If any part of the world is changing, the world as a whole is changing, and so, the change of the world as a whole is caused by something other than the world (the sphere of the heavens).)
  3. If the activity of the cause is itself a change or motion (which it is in the case of the spheres), then this per se effect must be caused by another prior, simultaneous cause.
  4. In a chain of per se or essentially subordinated causes, there would be no last effect (which there is) if something were not driving the whole chain (as first cause).  (Since there must be a first initiator of motion in essentially subordinated causes, the chain cannot be infinite.)
  5. Therefore, there must be a First Mover of the whole universe which acts and causes change, but whose causing activity is not a change or motion.

            Thus, having established the reality of something – the non-physical, non-moving but actual and active cause of motion, Aquinas identifies it according to what he already knows as a Christian:

VI. and this everyone understands to be God.

In looking for the explanation of motion Aquinas agrees with Aristotle that there must be something which is its ultimate cause.  Aquinas, however, is a Christian theologian, and is writing a work for beginners in theology.  He does not think that this demonstration for the existence of the ultimate cause of motion definitively proves all that Christians believe about God.  He merely has shown that there must be some real thing to explain motion, and notes that a Christian will recognize in this cause part of what he means by God.  He incorporates this proof into his work of theology, not because he or his readers is uncertain whether there is a God, but because it illustrates a truth about the God he believes in, namely, that reason can demonstrate God’s existence based on the existence and nature of motion. The non-Christian (or the Christian for that matter) might question whether the cause whose existence has been demonstrated is to be identified with God, but such a question does not affect whether the argument has shown that there is something there which explains why things are moving

Assessment of the First Way

Having examined closely what Aquinas says in the First Way, what he meant and why he thinks it’s true, it remains a fair question to ask whether the proof is successful.  That is, are the premises of the argument true, and does it prove its conclusion?  Aquinas’s argument clearly relies on Aristotle’s physics and cosmology for his understanding of motion and change, for these were the best scientific theories available at the time.

Since the time of Copernicus and Galileo, modern cosmology has shown that the earth does not rest at the center of a series of nested spheres whose combined rotation explains the apparent motions of the moon, sun, planets and stars.  But the fact that the earth and the rest of the planets orbit the sun does not by itself undercut the physical underpinnings of the First Way.  For the argument only requires that the changes and motions on the earth be the result of a common celestial motion which conceivably could be compatible with a system of moving causes centered on the sun instead of the earth.

Some have supposed that the modern understanding of motion discovered by Isaac Newton eliminates the need for a continuous moving cause to account for the continued motion of an object.  Newton’s First Law states that a body in uniform rectilinear motion or a body at rest will continue in motion or at rest until acted upon by an external force.  This is a rather limited, theoretical postulate about spatial displacement; no objects in the real world ever behave according to the motion (or rest, for that matter) described by Newton’s First Law.  It does postulate the continued spatial displacement of an object in the absence of a moving cause by attributing it to the object’s inertia. But the real value of Newton’s First Law is that it requires that there be a force operative for any observed change in state.  Stated negatively, it says that an object remains in its present state (in motion or at rest) unless acted upon by an external force; stated positively, it postulates that whatever changes state is changed by some external force.  Like the principle in the First Way, Newton’s First Law asserts that whatever forces must be at work to bring about the change of state are external to the thing whose motion changes.  Since, as we saw, motion for Aquinas is any change in state, this view essentially conforms to or fits under Newton’s First Law: any change (in motion or rest) must be brought about by something (some force).  Newton, like Aquinas, and following Aristotle, requires that what brings about the change must be other than what is changing.

There are, however, two fundamental problems in reconciling the First Way with modern physics.  First, it seems that premise (II) is not true: not all motion or change (even purely physical motion) is caused by the motion of something other than what is changing.  Rather, some things change of themselves through the exercise of intrinsic physical forces.  According to modern physics, all physical changes and motion are the result of the four fundamental forces: gravity, the weak and strong nuclear forces, and electromagnetism.  The objects or particles from which these forces operate bring about every physical change that occurs, either within themselves, or through interaction with other objects or particles. The motions of planets, stars and galaxies, as well as much of the natural motion on the earth are the result of the gravitational attraction between massive objects.  The motions and changes which result from chemical reactions are ultimately reduced to the bonds between atoms sharing electrons, and are grounded in the electromagnetic force. The strong nuclear force keeps subatomic particles within nuclei bonded together, and releases tremendous energy when those bonds are broken, in nuclear fission within stars, reactors and bombs.  The weak nuclear force brings about the radioactive decay of certain elements.  In all of these cases, the physical changes do not come about from the continual action of an external moving agent, as the First Way requires, but from the intrinsic capacities and tendencies modern physics identifies as fundamental forces.

Second, neither the individual physical motion or changes we observe, nor the motion of the universe generally, is the result of essentially subordinated chains. At most they are the result of temporally successive transfers of energy.  This fact is a corollary or implication of the fact that physical change and motion result from intrinsic forces and not from the continuous action of per se causes.  The motions we observe are displays of kinetic energy, but these are not the net output of simultaneously acting moving caused causes.  Rather, the energy is part of a universal system in which the energy has been transformed, and is being transformed (through further interaction), but the total energy of which is being conserved.  The sum-total of mass-energy in the universe is constant (or so it is believed) and the universe as a whole is a system of interchange between mass-energy in various states.  The universe generally does not behave like a system of simultaneous moving gears or a train of rail cars, necessarily driven by an engine.  Some parts of the universe we observe are in fact such systems, but our identification of these systems is relative to artificial frames of reference. Even the energy in such systems of simultaneous moved movers originated from the kinetic energy produced elsewhere on earth and can be traced back temporally through the formation of our sun and solar system, ultimately to the origins of the universe in the Big Bang.  Whether this event must have a prior cause or not, its cause would be ‘temporally,’ or at least, successively prior.  Even if all energy, and so motion, originated in the Big Bang (with or without a non-physical prior cause), this origin is not a per se cause, simultaneous with the motion we observe here and now, which is what the First Way requires.

So, the upshot of all this is that Aquinas is saying in the First Way that there are motions in the world, and that some of them are the per se result of simultaneously prior motions – the motion of the heavenly spheres. Thomas argues, rightly, for what it’s worth, that there could not be an infinitely long series of these moved movers, even though, as it turns out, there are not any spheres or any moving causes of all terrestrial motion (lumped together). In fact, no natural motion or transient alteration, it seems – neither gravity, nor radioactive decay, neither chemical reactions, nor electromagnetic actions, nor the energy stored and released by the strong nuclear force – none are the result of simultaneously acting moving movers.

As an example, gravitational motion – say the orbit of the earth around the sun – is caused by the mass of the earth and the sun, and arises from their mutual attraction. The earth’s orbital motion does seem to be a per se effect of (simultaneous and sustained by) the masses of these two bodies, but my point is that there is no motion that is the per se cause of the sun and earth being massive, or having mass. The cause of gravity just seems to be the nature of mass, and so to Aquinas and Aristotle, it would be a formal cause, not a motion. And the cause of a thing’s form is the efficient cause of the thing, which causes it to exist (by fashioning pre-existing matter or creating it ex nihilo (and sustaining the thing in being)). Thus, the gravitational motion is the last effect of a per accidens series which formed the sun and earth over time, ultimately beginning in the Big Bang (or something prior, or with no beginning). The present gravitational motion is being sustained in being, either as the finite or infinite per accidens series which is the universe, by a First Cause of Being. This account, while true, is the subject of the Third Way and the argument of De Ente et Essentia, Chapter 4.

This is how the First Way is usually explicated by those who at least have the virtue of recognizing it invokes a per se ordered series of essentially subordinated causes. They begin with an obvious case of motion (transient alteration) but then quickly identify a sustaining cause (gravity, or heat from the sun) and then show how that sustaining cause is itself sustained in being by a First Necessary Cause. In the case of Joseph Owens and John Knasas, they jump immediately to motion requiring a cause for its esse (to be), the First Necessary Cause.  Again, this is true – the First Necessary Cause is a cause of motion: a per accidens, not per se cause. But this ratio or kind of intelligibility which confines its focus to being and its proper cause is the approach of the Third Way, not the First.

As far as I can tell, all physical transient motions are per accidens effects of temporally prior efficient causes, which is precisely why Thomas thought, and most modern readers think, the material universe could be temporally infinitely old. For moderns, they conclude that there need be no first (per accidens) cause of motion, and in this they are right, whereas Thomas also thought that this per accidens series of efficient causes was sustained by per se moving causes (spheres); Thomas and Aristotle happened to be wrong about this. As it turns out, though, material things can either generate of themselves transient motion (gravity and weak force) or pass on energy in different forms, storing potential energy (electromagnetism and strong force) and releasing it by means of per accidens efficient causes. So, I conclude, the First Way is defeated by contingent facts about the actual material universe that modern science has discovered.

Aquinas’s First Way attempts to prove that there must be a first unchanging and motionless cause of change or motion in order to ultimately account for apparent motion on earth.  Given the physics and cosmology of Aristotle, he had good reason to think that the proof was demonstrative and successful. But now that we have better reasons to accept the contemporary understanding of physics and cosmology, it seems clear that motion is not always the result of the actuality of an object external to what changes, nor are changes ultimately the result of per se causes.  Since key elements of the First Way have been discovered to be false by modern physics, the proof is not successful.  It will have to await another treatment to investigate whether modern physics, which generally accepts the origin of the visible universe in a Big Bang, can generate a demonstration for a first cause of motion beyond physics.  If a proof can be made for a non-physical cause of the universe based on modern physics, the First Way of St. Thomas Aquinas is not it, nor can Aquinas’s argument provide the basis for it.

Revised 5/5/2021

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[1] What the starting point of the First Way is, and what Aquinas means by motion, has received a surprising variety of interpretations, notable among which is the ‘existential’ interpretation the First Way advanced by Joseph Owens and John Knasas.  According to this reading, Aquinas is pointing to the existence of the characteristic of motion, which must ultimately be explained in Self-Existing Being (which is how Aquinas understands God).  For a discussion of the textual warrant for this reading, see William Lane Craig, The Cosmological Argument from Plato to Leibniz, p. 168ff. [Return to text]

[2] Cf. Craig, p. 172-3. [Return to text]

[3] See Reply to Objection 2. [Return to text]

[4] Cf. ST Ia 46, 2. [Return to text]

[5] Meta XII, 8. [Return to text]

[6] Cf. ST Ia 46, 2 ad 7. [Return to text]

[7] Ibid. [Return to text]

[8] Thomas Aquinas: God and Explanations, p. 126ff. [Return to text]

[9] “Aquinas” in Three Philosophers, p. 113. [Return to text]

[10] Martin’s considerations, in fact, would render any motion, even celestial motions (see below) as lumped together with the totality of terrestrial motions.  But such a universal lumping together would obviate any need for Aquinas to argue against an infinite regress in moving movers, since all moving things (even moving movers) would have been lumped together with moving effects. If all causes which are themselves in motion were lumped together with the motion of the whole created universe, there could be no regress (finite or otherwise) of movers-in-motion since there is only one (lumped-together) motion of the universe.  Clearly Aquinas thought a regress was not only possible, but was actually finite.  He could not, then, have envisioned the universal lumping together Martin attributes to him, but only a terrestrial one that includes motions on earth. [Return to text]

[11] 1073b18ff. [Return to text]

[12] Oportet ergo quod caelum sit causa omnis motus in istis inferioribis corporibus. [Return to text]

[13] Phys. II, 2, 194b14. [Return to text]

[14] Videmus enim omnia quae moventur, ab alii moveri, inferiora quidem per superiora, sicut elementa per corpora coelestis, inferiora a superiora aguntur. Compendium Theologiae I, 3. [Return to text]

[15] Sicut videmus quod alterations, et generations, et corruptions, quae sunt in istis inferioribus, reducuntur sicut in primum movens in copus coeleste, quod secundum hanc speciem motus non movetur, cum sit ingenerabile et incorruptibile et inalterabile. Ibid, I, 4. [Return to text]

[16] Craig, p. 174. [Return to text]

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