Aquinas, Duns Scotus, and their followers on self-motion and the generation of new substantial forms

The classic argument against self-motion

From Saint Thomas’ First Way:

Now whatever is in motion is put in motion by another, 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. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. 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. Therefore, whatever is in motion must be put in motion by another.

Criticisms of the argument

A skeptic would respond as follows.  This principle of proportionate causality (PC–It’s called several things in the literature.  I took this name from Edward Feser.) doesn’t even work for Saint Thomas’ chosen example, in that cold objects can become hot through friction or exothermic chemical reactions without any prior thing being hot.  Whatever agent causes the heating must be in act in the sense that it must exist and have some power to influence the heated object, but it needn’t itself possess the actuality of being hot.  For that matter, an object can cool itself without being influenced by a cooler body, e.g. through blackbody radiation.  Thus, if Aquinas means that the cause must possess the form it actualizes in the effect, this conflicts with experience.  Nor is it obvious a priori why forms can only impress themselves and not causally connect to other forms.  Alternatively, one could propose a more modest PC principle, that the effected act pre-exists in the cause only in the sense that the cause possesses a power to effect the relevant change, but then the principle is tautological, and the argument against self-motion collapses.  It gives no reason why a being may not have a passive potency to receive x, an active potency to cause x (due to the possession of some other form), and may then act on itself without any need for the two potencies to reside in different parts.

The evolution and chemistry debates

Although the strong form of PC presents grave difficulties, it exerts a great deal of influence on the Catholic intellectual world, as seen particularly in discussions of the theory of evolution.  Recently, Fr. Michael Chaberek has written a book, Aquinas and Evolution, arguing that the evolution of new species is incompatible with Thomism because it violates this principle; at some point (if species are equivalence classes, as a Thomist will no doubt insist they are) parents must produce an offspring of a different species.  Fr. Nicanor Austriaco replies in Public Discourse, quite correctly, that the same argument could be made to prove that hydrogen and oxygen cannot combine to form water.  (That is, even a strong-PC advocate will allow that H and O can be material causes of existing water, but cannot allow that they can be efficient causes of water to be produced.  Note that Thomist holism actually makes the problem worse.  If molecules/compounds were nothing but their component atoms/elements, there would be no problem, but the current Thomist position is to grant full reality only to wholes and only virtual reality to parts.)  The conclusion is absurd, but how to avoid it within a Thomist framework?  Austriaco suggests (with precedent in Aquinas’ Aristotelian cosmology) that more perfect substantial beings may be involved, namely God or His angels.  Just as such beings can enable chemistry, so could they enable the evolution of new species, and before that the origin of life itself.  The appearances are saved, and we needn’t deny that chemical reactions happen, but look at the price!  We must invoke miraculous divine (or at least angelic) intervention to explain perfectly ordinary (actually, ubiquitous!) natural phenomena.  We have been led to a position not far from occasionalism.  Should this not cause us to reflect on whether the metaphysical principle that so consistently engenders outlandish conclusions is really certain?

Physically possible but metaphysically impossible?

Most people assume that the set of metaphysically possible processes is larger than and includes the set of physically possible (i.e. consistent with scientific laws) processes.  In some versions of Thomism, this appears not to be the case, in that the laws of physics and biology continually predict changes that are metaphysically impossible, and God (or His angels) must constantly intervene just to make the scientifically expected changes occur.  I have pointed this out before in the context of the origin of life and human ensoulment.  As Father Austriaco’s example makes clear, secondary causality really doesn’t extend very far at all with strong PC.

Again, it is possible that this is true, just as it is possible that full occasionalism is true, but it clashes violently with my intuition.  Fortunately, disciples of Aquinas aren’t stuck with it.  Thomists such as Dr. Feser point out that effects can exist virtually or eminently in their causes (basically allowing PC to become the tautological “The cause must have the power to produce the effect”).  But the extreme view is influential.

Duns Scotus on self-motion

The Catholic intellectual tradition as a whole has been open to wider possibilities of possible causality.

I’ve just been reading an interesting conference proceedings entitled Self-motion:  from Aristotle to Newton.  Many of the papers address the apparent conflict, which had been perplexing me, between Aristotle’s denial of self-motion in the Physics and his usual treatment of animals as self-movers.  The most fascinating contribution, though, is Peter King’s “Duns Scotus on the Reality of Self-Change”.  It seems that the Subtle Doctor put forward detailed arguments for the possibility of self-change within an Aristotelian framework.  Scotus painstakingly distinguishes different senses in which a potency or act can be in a subject, shows how common arguments against self-change confuse these senses, and that when all terms are used consistency much weaker conditions are obtained, which although not trivial do not rule out self-motion.  Scotus argues for what was then considered the least defensible case of self-motion, namely locomotion of inanimate objects, such as the elements naturally rising or falling.  Having clarified principles, Scotus believes causality can be univocal (an active form imprinting itself) or equivocal (an active potency to actualize a different form in a subject–possibly itself–with the appropriate passive potency).  A world with self-motion is, he thinks, more excellent and more unified (via causal connection between forms) than one without such powers.  Does this sink Aristotle’s argument for a prime unmoved mover from motion?  Scotus thinks not.  He thinks the argument can be formulated in terms of causes of the potency for change (including self-change) rather than the cause of change itself.  (Thus, the argument from motion would, he would say, be valid even in a stationary universe.)

Scotus’s work was just the beginning of a fruitful reassessment of Aristotelian physics in the 14th century, led by William of Ockham and Jean Buridan, which rejected the need to invoke the air or celestial bodies or angels to explain locomotion, replacing this with something like the modern concept of inertia, while Nicholas Oresme pioneered the mathematical study of motion.  It is unfortunate that Catholicism today has largely truncated its engagement with Scholasticism at the 13th century.


There are plenty of ways to accommodate Aristotelian principles to science and daily experience.  That’s not the problem.  The problem is that we need the problematic ones to do the necessary work in the argument from motion.  One could try to save the argument by switching at a key point from motion (in the Aristotelian sense:  activation of a form in a pre-existent substratum) to some more metaphysically basic category.  Duns Scotus does this by switching from motion to potencies for motion.  Dr. Feser does it in his new book on arguments for God’s existence by switching from motion to existence, a more basic kind of act.  In both cases, one finds oneself on stronger ground.  I would prefer to just admit that the classic argument from motion is invalid and drop it.

17 Responses

  1. You’re missing some words after “Most people assume that the set of metaphysically possible processes is larger than”

  2. Bonald, maybe I’m confused because I don’t know what you mean by “self-motion.” St. Thomas usually means “change” when he says “move.” He also knows that self-causation is logically impossible because for something to make itself begin to exist, it must already exist. A cause must come before or coexist with its effect. But in the self-causation cause, the cause and the effect would need to be exactly the same thing. So, the cause and the effect would need to exist and not exist in the same respect at the same time and that requirement implies a self-contradiction.

    I can change myself in some ways. For example, I can wheelchair from my bedroom to the kitchen. For me to do that, though, I need an essentially ordered causal series, a causal series where each cause except the most fundamental one gets its causal power from another cause.

    To show you what I mean, I’ll use Dr. Peter Kreefts’s example about a train with an engine, a passenger car, and caboose. When the engine pulls the cars, pulling power flows from the engine to the passenger car and from the passenger car to the caboose. Since neither car can propel itself, it needs the engine propel it.

    The same kind of thing happens when you play pool. to move a billiard ball, you hit it with your cue. That ball strikes another one that whacks a third one, and so forth because you began the series. So, the series depends on you in something like the way the train cars depend on the engine.

    An essentially ordered set of causes differs from a contingently ordered set of them. Since my brother Dave has a natural daughter, he and his wife gave her life. But his daughter can give birth to children of her own, even after Dave and his wife die, since their daughter has her own causual ability.

    But for either kind of causal seriies to work, you need need a cause that has underived causal ability and only God has it. That’s why each contingently ordered causal series presupposes a hierarchical one.

    I like to surprise peeople who say that fossil record gaps undermine the theory of evolution. To do that, I tell them that if there were no God, there would be no gaps because there would be no one and nothing at all.

  3. We are using the word “move” in the same way. According to Aristotle, motion involves the actualization of a form in a persisting substrate.

    Aquinas ignores the different senses in which a thing may possess a form, which leads him to absurd conclusions such as that only a hot object can cause heat. As Duns Scotus points out, there’s no reason the same object cannot possess and active potency for instilling X and a passive potency for receiving X. Apart from self-motion, it can be the case that the act of X may be imparted to a substance so long as some object already has the active potency for instilling X, even if prior to this activation no object in the universe possessed X.

    The more sophisticated Thomists will grant this, since otherwise their philosophy would conflict with experience in many obvious ways, but the simple and false claim that beings can only impart forms that they actually have still crops up from time to time.

  4. in that cold objects can become hot through friction or exothermic chemical reactions without any prior thing being hot… Thus, if Aquinas means that the cause must possess the form it actualizes in the effect, this conflicts with experience.

    Fortunately, this is not what St. Thomas means. He is not saying that the cause must itself be hot. After all, God is not a subject in which “hot” can reside as an attribute.

    He is saying that the cause must have an actuality that either is like the effect OR is an actuality HIGHER than the effect, of which the effect is a lesser sort of expression of the actuality. The exothermic reaction is a great example, because the heat is implicit in the stored energy of the chemical bonds. That is to say, the energy (the actuality) of heat was present in a different way than as “hot”, and by reason of the exothermic activity that energy came to be expressed as “hot”. St. Thomas talks about this principle all the time in other contexts: God knows things, but he DOES NOT know them in the manner we know them, he knows them in a higher mode of knowing. He is able to cause (through many intermediaries) our knowing not because our mode of knowing is just like his, but because our mode of knowing is a kind of actuality of lesser fullness than his, not more. Likewise, God has implicitly the actuality of sensation as a good, but he doesn’t have a sensation itself because he does have a body. He has the actuality in a higher way.

    The “through friction” is via an agency of motion, wherein the energy of motion is transferred as kinetic energy in the (formerly) cold body – one can hardly use this as an example to discommode Aquinas.

    Alternatively, one could propose a more modest PC principle, that the effected act pre-exists in the cause only in the sense that the cause possesses a power to effect the relevant change, but then the principle is tautological,

    St. Thomas is asserting what Aristotle asserted, that “the power to effect the relevant change” JUST IS having an actuality of either the SAME form, or a HIGHER form that includes the lower implicitly or virtually. You would have to provide a counter-example where the cause does not have the “power to effect the relevant change” in virtue of having an actuality that is in some sense the same as the effect, either in the same mode or some higher mode of being.

  5. pilgrim,

    Thank you for your response. I don’t think “in the same form or greater” gets Thomas out of trouble here. There are also endothermic reactions, after all. I can easily manufacture many examples of change going both ways, since the laws of nature, insofar as we know them, are nearly time-reversal invariant, so nearly every process has a reverse process. An example medievals would have known is conversion between light and heat: sunlight warms, and sufficiently hot bodies visibly glow.

    As you indicate, Thomas’ metaphysics considered in its entirety is certainly not wedded to any crude version of proportionate causality. (I question, though, whether the more sophisticated versions do the work needed for the argument from motion to go through.) In fact, Thomas was much less dogmatic about this than his followers! He believed in the spontaneous generation of life for the perfectly logical reason that it appeared to regularly take place.

  6. I’ve been quietly following this series of related posts, and have been paying more attention to the whole Thomism/chemistry/biology affair which as arisen (or rather, re-arisen) lately. My immediate gut reaction was that works like those of Frs. Chabarek and Ripperger inadvertently do more harm to Thomism than to biological evolution. Maybe it’s naïve on my part, but it seems intuitively wrong (or at least, odd and eyebrow-raising) to try and disprove empirical observations using metaphysics. The intellect is first in the senses, and so on. It’s hard to weasel your way around different fossil presences at different geological eras, regardless of the exact theory/process you use to postulate why this is the case. Criticising the philosophical baggage attached to, say, doctrinaire Darwinism (as an “-ism”), or criticising it as a theory, among others, to explain evolution, sure.

    I view an A-T metaphysical system as generally correct, or at least, the most correct given other options. However it’s clear that some things need to be ironed out. Similarly, following Slumlord’s comments on an earlier post, I am forced to concur that this whole affair has made me warier about capital-R capital-T Rad Trads, despite being what a “normal” Catholic would consider somewhere between Torquemada and Uncle Adolf. When I read an article by someone who is convinced that the real reason the Church is in crisis is that she failed to dogmatically proclaim YEC, I am forced into a position where the general credibility of the “trad” faction is in doubt, despite being a retrograde bigot otherwise sympathetic to it.

  7. I managed to dig something up which may be of (ok, definitely IS) of interest here. A paper which responds specifically to the issue of the generation of new substantial forms, even from parents which lack that substantial form, and responds to the type of objections raised by Frs Ripperger and Chaberek (quoting the former directly).

    Definitely give it a thorough read. I could quote the conclusion, but it would be to open to misunderstanding without considering the arguments put forward in favour of substantial form necessarily following from the correct disposition of matter. He also, in my opinion, manages to resolve rather well the whole problem of “occasionalism”, and addresses that problem/accusation explicitly.

    Click to access and-man-became-a-living-being.pdf

  8. […] potency, causality, etc (and those who try will often reach crazy conclusions, such as that all of chemistry is miraculous) and therefore cannot be confident that we truly understand them.  We will have succumbed to the […]

  9. […] potency, causality, etc (and those who try will often reach crazy conclusions, such as that all of chemistry is miraculous) and therefore cannot be confident that we truly understand them.  We will have succumbed to the […]

  10. I think this could be fixed by adopting an idea of causation where we say that the cause itself must be actual, and the cause itself must be capable of the effect, and that in order to actually cause the effect the cause’s capability must be actualized, and that is the only way in which the cause and the effect must be the same kind of thing.

    I don’t claim that’s Thomist, because it’s been too long since I read the Summa seriously, but I am curious if such an admission would get us into trouble.

    At the very least, this admission would make it harder to be certain that a given cause is the cause of a given effect than if PC were true.

  11. I agree with your revised law of causation. It won’t do the work that some scholastics want from a causation principle, but that’s work that I don’t want done anyway. My quarrel with some modern-day scholastics is that their principle means one (modest) thing when they’re defending it and another (contentious) thing when they’re using it to disprove evolution or whatever.

  12. I think you could use the modified PC statement to work as a disproof of, if not evolution wholly, then the evolution of sapience. But you’d have to do the hard work of showing that intelligence and blind complexity are different categories of thing.

  13. @ Bonald

    Feel free to delete this comment. It’s just a heads up to let you know that Zippy has died in an accident earlier this week. His son confirmed it on his blog.


  14. I held these comments at first, but since I’ve seen the announcement at Zippy’s and Dalrock’s, I guess the news is out. Thank you for letting me know. Sad news indeed.

  15. […] that the principle of inertia is not necessarily a problem even if one limits oneself to Thomism. (Dun Scotus had no problem with self-motion.)  The goal, though, is not to defend scholasticism but to […]

  16. […] that the principle of inertia is not necessarily a problem even if one limits oneself to Thomism. (Dun Scotus had no problem with self-motion.)  The goal, though, is not to defend scholasticism but to […]

  17. […] (Similarly, metaphysical principles that come out of nowhere to do theological work, such as the principle of proportionate causality, which otherwise only makes nuisance for us as we try to explain away all the obvious […]

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