I've finished with Ed Feser's treatment of Aquinas' First way in Aquinas, but he did not satisfactorily (or intelligibly for me) address my question: How do we know the unmoved mover for any given change must be single and lack potential?

He gives what looks to me as "a hand-waving argument", i.e. skipping a step assuming the reader sees what he's getting at:

Consider how [a hand moving a stick to move a rock to move a leaf] would have to continue beyond the point at which we left it, with the hand's potentiality for motion actualized by the arm, [...] the muscles' potentiality [...] by the firing of certain motor neurons, and so on [...] [T]his depends [on] the nervous system, which depends on [molecules, electromagnetism, gravitation, the weak and strong nuclear forces, etc]. To account for the reduction of potency to act in the case of the operations or activities of the hand [etc] we are led ultimately to appeal to the reduction of potency to act vis-a-vis the existence or being of ever deeper and more general features of reality; for "it is evident that anything whatever operates so far as it is a being" (QDA 19). But the only way to stop this regress and arrive at a first member of the series is with something whose very existence, and not merely its operations or activities, need not be actualized by anything else. This would just be something which, since it simply exists without being made to exist by anything, or is actual without being actualized, is pure act, with no admixture of potentiality whatsoever. For suppose it had some potency relevant to its existence (its existence being what is relevant to its status as the end of the regress as we have continued it). Then either some other thing actualizes that potency, in which case we haven't really stopped the regress after all [...] or some already actual part of it actualizes the potency, in which case that already actual part would itself be both pure act and, properly speaking, the true first mover. Now, having no potency to actualize, such a being could not possibly change or move. Thus [...] not only unmoved, but unmovable.

The part I've bolded is where I get lost: What does he mean by 'deeper and more general'? I suppose by 'deeper' he means 'smaller', such as the existence of quarks. By 'general' I suppose he means how the Coulomb force appears the same everywhere in space. Yet why must this lead to God, rather than stop at the material level? Why can't the quark be a necessarily-existing being not deriving its existence from anything else? Why can't the constancy of atomic forces be a brute fact just as "God is love" (or God's nature is love) is a brute fact? I do not see why this per se series of movers must terminate in something beyond our physical universe, and I don't think he clearly explains this point: He merely asserts it.

Moreover, I don't see that his argument that the unmoved mover must lack any potential is true, and I think I even have an argument proving it false: A metal beam could have wet paint on its surface whereas it has the potential to change temperature. Whether it's at 20 C or 25 C is irrelevant to its causal ability to smear paint on something else that comes into contact with it. The metal beam with wet paint is not the unmoved mover of a ball getting paint on it because of course it depends on its atomic structure remaining intact, but since the only difference between an instrumental mover (one that is actualized by another) and the ultimate unmoved mover is whether a potential with respect to the observed change is actualized by another, it appears possible that the unmoved mover of that series could have potential regarding some other interaction.

So we can conceive of an ultimate mover that has some potential with respect to something else, other than what it happens to be moving at that time. Returning to Feser's hand-flexing example, it appears to me plausible that the electromagnetic and atomic forces, together with the quarks, could be a collection of unmoved movers, even while the electromagnetic force has the potential to be overwhelmed by the gravitational force (suppose a massive object like a black hole is moving towards the flexing hand), i.e. its effects cease to be observed because the quarks instead move due to gravitational influence. (I need to check my physics, actually, because I think the strong force is billions of times stronger than the gravitational force -- but I think we can conceive of a thought experiment in any case where the per se series could be thwarted by the introduction of another entity, and then it would seem the unmoved mover has some potential with respect to that previously-absent entity, such as injecting another quark into a molecule changing its atomic makeup, changing the physical forces acting thereon.)

Would you please specify my error and explain how I'm wrong? Why must a per se series terminate with a cause that lacks any potential, rather than merely lacking potential relevant to the observed cause? Would you please elucidate Feser's argument for me if he is correct?

  • 2
    Was this meant for the philosophy stack? I understand the context but the discussion is purely in the realm of philosophical debate. The Bible is more concrete, it makes absolute statements which evidently conflict in some cases with modern science (why I question the science not God). So I guess I’m asking if you would be satisfied with a response that consisted of Bible verses or if indeed you sought to have a philosophical response about the metaphysical in a generic generalized sense, without specificity to the God of the Bible?
    – Autodidact
    Jun 23, 2019 at 20:30
  • Yes, it is a philosophical question without presupposing the inerrancy of sacred scriptures. I thought philosophical questions were included insofar as they related to the existence of God, as particularly implied by the tags "st-thomas-aquinas" and "existence-of-god". For the tags I've used to exist yet this question be considered off-topic would strike me as very strange, as these tags do indicate the essence of this question. Jun 23, 2019 at 20:34
  • See Dr. Feser's 2019 Aquinas Series lecture "Classical Theism and the Nature of God," esp. toward the end where he discusses arguments for monotheism from those of divine simplicity.
    – Geremia
    Jun 23, 2019 at 22:29
  • I’ve not suggested it’s off topic. I asking if you would accept only one form of response, philosophical.
    – Autodidact
    Jun 23, 2019 at 22:32

2 Answers 2



This distinction arose from the Parmenides objection. Let me summarise the argument why the change is not possible and then I will resolve the argument.

Proposition (Parmenides): Change is not possible.


  1. A thing either is or is not.
  2. If it is, it is being; if it is not, it is non-being or nothing.
  3. Change requires that something which is not, comes to be.
  4. When change occurs, that which comes to be, before the change either is or it is not.
  5. If it is, then it is being and if it is not then it is non-being.
  6. We can not say that before the change it is (or that it is being), because that would mean that it comes to be what it already is; a thing can not come to be what it already is (that is contradictory to the whole point of change).
  7. In the second case, it is also impossible that before the change it is not (or that it is non-being) because that would entail that non-being becomes being.
  8. Therefore change is not possible.

However, Aristotle resolved this problem by admitting that there is the division inside being, namely, that something can be being in act or being in potency. He finds the middle ground not between being and non-being, but between being in act and non-being and this middle way is called being in potency. So the change does not require that non-being comes becomes being, but that being in potency becomes being in act. Hence we have the first Thomistic thesis:

Potency and Act divide being in such a way that whatever is, is either pure act, or of necessity it is composed of potency and act as primary and intrinsic principles.


  • A man is potentially a musician.
  • A metal bar is potentially hot.
  • Wood is potentially of green color (if painted in that color).


Terms: Change consists formally in the transition from one way of existing to the other way of existing; i.e. reduction of the potential state to the actual state. In every real change we need to distinguish three things:

  1. Subject who is being changed,
  2. initial state from which change occurs (terminus a quo) and the final state to which is the end of change (terminus ad quem),
  3. the transition itself which itself constitutes change formally.

Example (Heating of cold metal): Subject which is being changed = the metal. Initial state = cold metal, final state: hot metal. Change = heating a metal.

Terms: In philosophical terminology we distinguish between physical changes and metaphysical changes and we distinguish between changeable being and unchangeable being and we define motion.

  1. Physical changes = those changes who have one term positive (or existent), ie. those changes where the existent subject gains or loses something (like in the previous example). All substantial, accidental are physical changes (change of motion is also a physical change)
  2. Metaphysical changes = those changes who have one term negative (non-existent), ie. these changes are creation or annihilation. This kind of change is not really considered a change at all, but rather creation is something different than change. Change requires that before and after the change there is something common (ie. the subject which is being changed), but when creation occurs, the initial state is nothing and the final state is something, and they do not have anything in common and that is the reason why creation is not considered to be a change at all (similarly the annihilation is also not considered to be a change).
  3. Changeable (mobile, mutabile) being = every being which can go from one way of existing to another way of existing, ie. where some potential state is reduced to the actual state.
  4. Unchangeable (immobile, immutabile) being = that which excludes the possibility of transition from one way of existing to another way of existing.
  5. Motion = reducing being in potency to being in act.

Proposition: There exists a first unmoved mover.


  1. There is a (physical) change in the world (note that we start not from changeability of the world but from changeability in the world).
  2. Whatever is moved is moved by something else.
  3. That something else is either moved or unmoved.
  4. If it is unmoved we have our conclusion.
  5. If the mover is itself moved then we need to pose yet another mover.
  6. Again if that mover is unmoved we have our conclusion and if that mover is itself again moved we need to pose yet another mover.
  7. This can not go to infinity.
  8. Therefore we need to pose the first unmoved mover.

I am not going to explain every step now but only if one of these steps is not clear to you (you can pose an objection in the comment section). The reason is that this part is not essential to your question.


Proposition: If something is composed of act and potency then that subject can exist in different ways.


  1. If some subject X is composed of potency (more precisely: an object has an admixture of potency), that means that X is potentially existent in some other way which is different then the way it exists right now (namely in a way which includes that potency).

  2. That means that there are at least two ways in which X can exist.

The point of the just now proved proposition is this: as soon as you admit potency in your subject, you have admitted different ways in which that subject can exist. We will illustrate the proposition with the following example.

Example. Say some metal bar is now cold. The metal bar has the potency to become hot. Therefore, there are at least two ways in which a metal bar can exist, it can exist as a cold metal bar, or as a hot metal bar.

Also, it is trivial to see that a reverse direction of a previous proposition is also true. For if something can exist in many different ways then it is potentially existent in that way, and potentially existent in another way, etc.

Key insight: Subject that can exist in different ways (this is true for subjects composed of potency) is in itself indifferent to existing in any particular way. For if it was not, then it would necessarily exist in one way and the other ways of existing would really not be possible for that object. The subject which is in itself indifferent for existing in any particular way must have a reason outside itself which determines to one particular way of existing. I will use an example to illustrate the point.

Example: A metal bar can be cold and hot. Metal bar by itself is indifferent to either, therefore there must be a reason outside of the metal bar which determines the metal bar for being cold or hot, namely, the temperature of the atmosphere (or whatever).

Proposition: First unmoved mover must be also a pure act (ie. unmovable).


  1. Let us suppose that an unmoved mover has some potency.
  2. That means that he can exist in different ways.
  3. Also, he by himself is indifferent to existing in any of these ways; for if it was otherwise he would exist in only one way which would imply that there is no potency (and that would be a contradiction with the premise).
  4. Because we have concluded the existence of the first unmoved mover he exists in one particular way.
  5. But because he himself is indifferent to existing in any particular way, there must be a sufficient reason which determines him to one, ie. another mover which actualizes his potency to exist in that particular way.
  6. Now we have a contradiction because then the first mover is not really the first mover but needs something else to move.
  7. So we can conclude that the unmoved mover is void of any potency.

So the gist of it is that an unmoved mover must be also unmovable to account for motion in the world (because first unmoved movere which is not pure act would be a contradiction as I have just shown).

I will edit my answer more later.

  • Thank you for the great answer, but I have a problem with your Pure Act proof statements 2, 4, 5: Potential can be actualized in a per se series (metal becoming hot) or a per accidens series (metal is broken by a bomb; remains broken after explosion ceases), and the unmoved mover (UM) only needs to be unmoved while serving as the UM of the change currently being observed. You write as if UM is not capable of change after the change it's causing has finished, but it is conceivable that the UM could have its potential actualized ending the change (e.g. chemical reaction) or after the change. Jul 6, 2019 at 3:18
  • I think I should (or must) be more direct and say that premise 5 is false: It could be a brute fact from a previous per accidens series that the unmoved mover is currently in the state it's in to cause the observed change, and it might change later if it comes into contact with some other mover. Why is it not possible for the unmoved mover to be in its current state as a brute fact from a previous per accidens series? Jul 6, 2019 at 3:30

RE falsity of premise 5: The point of the proof is that if some thing 1 makes the putative first mover (thing 1') be the way it is, that thing 1 is really the first mover of whatever changes are laid to the putative first (1'). That thing 1 is the first mover really; and it cannot be made to be the way it is by some other thing 3 or else it is merely putative also.

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