In the first way of Aquinas it is proved that there exists an unmoved mover (or unchanged changer or even better, actualizer which was not actualized). It is often claimed that God is pure act without any potency; but I seem to think that in the first way it is proved that first mover is not moved; but it was not proved that first mover is unmovable.

Why does the first mover need to be unmovable? Why can it not have some potencies which are not ever actualized?

  • Why can’t God be pure potency instead? This would be the only actualiser not actualised - an unmoved mover. There is an assumption that God must be ‘actual’ in order to exist. Jan 21, 2019 at 3:22
  • @Possibility Pure potency would not really exist. If something does not exist it can not do anything. Then also, it can not actualize anything.
    – Thom
    Jan 21, 2019 at 4:22
  • But didn’t Aristotle say that in order to exist, it must either exist actually or exist potentially? So something that exists potentially still exists. There is a tendency to leap from ‘exist potentially’ to ‘would not really exist’ to ‘does not exist’ without qualification. Jan 21, 2019 at 23:52
  • @Possibility I agree that what exist potentialy is real. But it would seem to me that potentiality can exist only insofar as it is in act (potency of ice to melt only exists if we have ice existing in act, and then it has potency to melt). What would it mean that potency that ice melts exists without there really being ice existing in act?
    – Thom
    Jan 21, 2019 at 23:57
  • You’re confusing the actuality and potentiality of two different events: to melt and to exist. The act of melting can exist for ice only if there exists for ice the potency of melting. Likewise, the actuality of existing for ice is contingent upon the potentiality of existing for ice. The problem is that we examine a single, limited potency for an entity as if that were potentiality. But ice has much broader potentiality than simply to melt, and most of that potential is not exclusive to the existence of ice, but actualisable by many more elements in the universe... Jan 22, 2019 at 3:46

5 Answers 5


The philosophical axiom that

something cannot give what it does not have
nemo dat quod non habet
(cf. I q. 2 a. 3)

applies here.

If the First Mover were movable (i.e., not pure actuality, actus purus), it would lack something (for example:* the ability to move other things), and it could not give what it lacks to others (in the example: it could not change others). This is a contradiction (in the example: because we do observe that things change). Thus, the First Mover must be immovable.

*Any (absolute, not relative) perfection could be given as an example of what the First Mover could lack, and the argument would remain. Other examples: immateriality, omniscience, justice, omnipresence, etc.:

  • If the First Mover lacked immateriality, it could not give immateriality to creatures; but, we know that there are immaterial creatures; ergo….

  • Or: If the First Mover lacked all knowledge, it could not give knowledge to creatures; but, we know that there are creatures with knowledge; ergo….

  • Or: If the First Mover lacked justice, it could not give justice to creatures; but, we know that there are creatures with the virtue of justice; ergo….

  • Or: If the First Mover were localized to one place, it could not make creatures localized to no place; but, we know that there are creatures not localized to one place (e.g., purely intellectual beings, the angels); ergo….

  • Etc.

  • Is this structure of your argument: If the First mover were potential in some way it would lack something. Let us say he lacks A. If he lacks A he can not give (or do) A to others. But he gives (does) A to others. Thefore he does not lack A. How do we know he does not lack some B for which we can not say: but he gives (does) B to others? And this may be stupid question but what if we set for A=five fingers, ? I think I dont get the argument.
    – Thom
    Jan 19, 2019 at 20:37
  • @Thom Having five fingers (per hand) is a relative perfection (for humans), but it's not an absolute perfection; thus, it can't work as an example.
    – Geremia
    Jan 19, 2019 at 21:06
  • This question still remains: How do we know he does not lack some B for which we can not say: but he gives (does) B to others? Your argument if it is good seems to work only for perfections which we can observe in the world (because only then we can say: "we know ...)", for example knowladge. And it seems that you are saying that only some things we can substitute for A (namley, only absolute perfections). What are those and by what criterion can I say that something is absolute perferction and not just relative perfection?
    – Thom
    Jan 19, 2019 at 21:15
  • @Thom Perhaps you're wondering why immutability is a perfection?
    – Geremia
    Jan 19, 2019 at 23:29
  • 3
    @Thom ~1:00:50 of Dr. Feser's Aquinas Series lecture "Classical Theism and the Nature of God" reminded me of your discussion here. He discusses how "roundness, redness, etc.", because they exist in the world (God's effect), must exist in God (the world's cause) (∵ effect≯cause), not as they exist the physical world, but "as concepts or ideas".
    – Geremia
    Jun 20, 2019 at 23:09

See Aristotle Physics, Bk.VII :

[242a50] Since everything that is in motion must be moved by something, let us take the case in which a thing is in locomotion and is moved by something that is itself in motion, and that again is moved by something else that is in motion, and that by something else, and so on continually: then the series cannot go on to infinity, but there must be some first mover.

Thus, Aristotle's argument is based on the rejection of an infinite regress.

Regarding Aquinas, see Divine Simplicity :

According to the classical theism of Augustine, Anselm, Aquinas and their adherents, God is radically unlike creatures in that he is devoid of any complexity or composition, whether physical or metaphysical. Besides lacking spatial and temporal parts, God is free of matter-form composition, potency-act composition, and existence-essence composition.

See Augustine, The City of God, Bk.XI, §10 :

There is, then, a Good which alone is simple, and therefore alone immutable, and this is God.

Thus, if God is immutable, he cannot have "potentialities" that are not actualized.

  • This gets us to first unmoved mover. But how does this prove that first mover has no potency at all? Why can't he have potencies which are never actualized?
    – Thom
    Jan 19, 2019 at 15:13
  • @Thom "potentialities that are not actualized" doesn't mean 'no potency at all'.
    – Bread
    Jan 19, 2019 at 16:25
  • @Bread That is preciasly the point. Why can not we say that argument leads us to unmoved mover who has some potencies which will never be actualized? Does the argument somehow forbidd that possiblity?
    – Thom
    Jan 19, 2019 at 16:33
  • @Thom I can only presume that it is because it is eternal and infinite; and that it (imo) pervades and interpenetrates every part of the universe and also extends beyond time and space.
    – Bread
    Jan 19, 2019 at 16:49
  • Why is it that in 2019 Aristotle is referenced? There's has been much more advancement in our knowledge of the world since his lifetime for example that there is motion of particles even at absolute zero, uncertainty principle, etc.
    – Cell
    Jan 19, 2019 at 18:12

Here is the question:

Why does the first mover need to be unmovable? Why can it not have some potencies which are not ever actualized?

This answer only tries to address the first question hoping the second will fall in place.

One way to view this is to see the first mover as not a kind of body that moves other bodies such as one might think of a billiard ball moving another billiard ball. If it is not a body, the idea of some body moving it is not meaningful. Hence one should expect this immaterial reality to be unmoved.

Initially this seems odd until we remember that we don't think of the billiard ball actually coming in contact with the other billiard ball. What contacts both balls are immaterial fields that move or change both balls in some way.

Although the actual existence of these immaterial fields is often taken for granted, some physicists do question them as being real. For example, on page 42, Marc Lange quotes Alfred O'Railly's 1965 textbook, Electromagnetic Theory:

The assertion [of the electromagnetic field's reality], taken by itself apart from the quantitative force-law is scientifically otiose....It is merely the physically irrelevant statement of a metaphysical conviction....This is certainly not a legitimate physical theory at all; it is the confusion of metaphysical belief with metrical physics....

The question of the existence of unmoved movers can be viewed as analogous to questions about the reality of fields.

To carry this further consider what it means for there to be a "first" such unmoved mover. Dominic O'Meara describes Plotinus going beyond Plato's Forms to assert the reality of the simpler One as similar to what a scientist does when looking for a simpler, more unifying, explanation: (page 46)

Throughout the history of philosophy and science can be found the idea that everything made up of parts, every composite thing, depends and derives in some way from what is not composite, what is simple. This idea might be called the 'Principle of Prior Simplicity'. The Principle of Prior Simplicity has not lost its appeal: it still inspires scientists in their efforts to reconstruct the generation of the elements from an earlier, simpler state of the universe, and similar explanatory patterns, deriving the complex from what is less complex, can be observed, for example, in biology.

Use of terms like actualities and potentialities may specialize the description of unmoved movers toward particular philosophies. Shifting this analogically to fields may offer different descriptions and suggest other reasons to justify belief in unmoved movers.

The existence of such reality as immaterial unmoved movers and fields is questionable but if they are not real, as Alfred O'Railly insists they are not, some other explanations for the motion of bodies would be required.

Lange, M. (2002). An Introduction to the philosophy of physics: Locality, fields, energy, and mass. Blackwell Publishing.

O'Meara, D. J. (1995). Plotinus: an introduction to the Enneads. Oxford University Press.

  • When I say unmovable I mean unchangeble (even in principle), or better to say, without any potency. "If it is not a body, the idea of some body moving it is not meaningful"; well, angels have potency yet they do not have body. So, I am not sure that I understand how that answers my question.
    – Thom
    Jan 21, 2019 at 13:22
  • @Thom Those angels could be viewed analogously to fields or Platonic Forms and I agree they do have potency. But they also aren't the first unmoved mover nor are they Plotinus's One. This is why Plotinus needed the One (if I understand O'Meara). Plato's Forms were not simple enough. They still had "potency". They still contemplated something besides themselves. Jan 21, 2019 at 13:43

It has been postulated by some philosophers that God is the Universe.

Theologians have claimed that God is unmovable by any force other than itself.

Of course it can move itself, as it is dynamic and said to be omnipotent.

There are certain basic properties however which in God never changes.

That it is eternal and infinite, for example, may never change. In that respect, it is immovable.

Therefore if God is the universe, i.e. Reality itself, if anything had power to change it, everything -- reality as we experience it, perhaps even all consciousness -- would cease to exist.

There would be nothing left, only Death.

Therefore it must be immovable by anything other than the totality of itself. That is why it is called the immovable mover.

That is not to say that it cannot respond to acts of freewill, by humans for example.

Let us imagine hypothetically that humans decide to destroy their own habitation, Earth. Whether or not they succeed, the Universe will be affected by and will respond to such activity.

But although humans may be able to destroy the Earth, the Sun, the Moon, our own solar system, galaxy, and even ourselves as a species, we cannot destroy the Universe, no matter how hard some may try.

Meanwhile the universe can and does have freedom to move (or change) itself. Perhaps it expands and contracts like an enormous heartbeat, producing (or creating) "Big Bangs" and "Black Holes" and "galaxies" and other interesting phenomena.

Surely God can quite easily heal itself and recover from any and all assaults on it by certain members of the human species who, whether intentionally or foolishly, choose to destroy themselves and others.

And if not, then eventually everything would simply cease to exist and be replaced by death or nothingness.

An idea which unfortunately doesn't seem to bother some people.

However Justice (universal justice) is supposed to prevent that from happening, suggesting that if it indeed cannot happen -- if nothing can destroy the universe -- then God really is the immovable mover.


Aquinas is just forcing conclusion

You must remember that Aquinas was first and foremost a priest. Therefore, he could not just suddenly "discover" that idea of God (especially Christian idea of God) is contradictory to logic. Instead, he was trying to come by reason to the same conclusions other came by faith.

We must now ask ourselves why God must be unmovable. Something that is moveable is by definition changeable. Notion that God could change was terrifying for theologians : this would mean that God's plan could change, truth could change (truth is what God makes true), even teachings and prophecies from the Bible could be invalidated - definitely not a good thing.

Aquinas, as a Christian priest, could not accept idea of world(s) being in perpetual (uncaused) motion, therefore he accepted Aristotle's idea of first mover. He also accepted idea that something could not give what it lacks, which is a bit shaky, as certain qualities could be transformed (for example gasoline transforms it's chemical energy into kinetic energy of a car, i.e. movement). Therefore, he had a problem how would his first mover, being unmovable God, make other things move.

He "resolved" this this problem by putting God completely out of physical time and space, making him a category of his own. Aquinas makes God act with his willpower (Pure Act) , without needing any physical attributes for that act like other beings and objects. These physical attributes are called potencies in his works, somewhat similar to potential energy in latter physics.

  • 1
    Aristotle believed in an eternal universe, yet he believed there is First Mover. How does Aquinas's belief in a non-eternal universe relate to whether the First Mover must be immovable?
    – Geremia
    Jan 20, 2019 at 2:14
  • @Geremia Aquinas respected Aristotle as authority (he really didn't have much alternative for this), but didn't accept all of his concepts . First mover is very convenient idea for Aquinas, because this could be his God. I already explained why God must be immovable. Aquinas however didn't agree with Aristotle in regards of "unmoved mover's" inability to influence our material world.
    – rs.29
    Jan 20, 2019 at 13:24
  • he really didn't have much alternative for this <-- this is an odd claim. Are you familiar with the situation at the university of paris around the time of Aquinas?
    – virmaior
    Jan 21, 2019 at 14:54
  • You must remember that Aquinas was first and foremost a priest. Therefore, he could not just suddenly "discover" that idea of God (especially Christian idea of God) is contradictory to logic. This is an interesting claim, but it needs a bit of revision. If you added "committed to orthodoxy" after "priest" then it would make sense. There have been plenty of priests who weren't really committed to this sort of thing, so it doesn't automatically follow (even then) that a priest had to believe things that guaranteed orthodox conclusions.
    – virmaior
    Jan 21, 2019 at 14:56
  • @virmaior Western universities had very little texts from ancient Greek philosophers, mostly Latin translations. . Therefore Aquinas relied on what he had, and didn't have much choice : en.wikipedia.org/wiki/Transmission_of_the_Greek_Classics
    – rs.29
    Jan 22, 2019 at 4:28

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