Potentiality and inertia

A key thing to appreciate about potentials is that they are indifferent to what is actually the case. It’s because of this that they are able to play the role they do in accounting for the reality of change, together with actualities.

I have the potential to sit down even when I am standing up, and it is this potential that I actualize when I do eventually sit down. If I did not have that potential, then I would not be able to actualize it in myself and therefore not be able to sit down. We can see this work itself in cases where things lack the relevant potentials: a rock depends on other things to move it because it does not have the potential for self-movement, a squirrel never actually thinks about the physical laws of the universe because it lacks any potential for rational thought, and a match never produces snow when struck because it does not have any corresponding potential for this.

I also retain the potential to sit down while I’m actually sitting down, since I can’t be actualizing a potential I don’t have. This can be missed because we sometimes speak of the potential to become actual in some way rather than the potential to be actual in some way, even though the latter is more fundamental. This ambiguity is not particularly surprising, since potentials are capacities for being actual in particular ways and we see this ambiguity in other examples of capacities: a one-liter bottle has the capacity for containing one liter of liquid even when full — since that’s exactly what it’s doing — it just doesn’t have the capacity for containing another liter of liquid. Similarly, if I’m currently sitting down I still have my potential to be sitting down, even if it is no longer the case that I can move to the state of sitting down.

Thus, the potential exists in me regardless of whether it is actualized or not, and so, as we said, the potential itself is indifferent to what is actually the case. It follows from this that every potential depends on some other actuality in order to be actualized from moment to moment, and in an indirect way it also depends on some other actuality in order to be unactualized. At any given moment, the existence of a potential cannot guarantee one way or the other what is actually the case — it can only determine what could be, not what is the case. And this can’t be addressed simply by adding another potential into the mix, because that will suffer from the same limitation. Rather, what is needed is an actuality which either actualizes the potential or indirectly unactualizes it by actualizing some other incompatible potential, as sitting down is incompatible with standing up. Of course, this other actuality could itself be an actualized potential, and so on, and so on.

The indifference of potentials is, I think, the core reason for why actualized potentials need to be continually actualized by some other actuality. On the face of it, however, the result that potentials depend on actualities in order to be continually actualized seems to be at odds with the Newtonian principle of inertia. Since inertia is a well-known phenomenon, and since it makes our result counter-intuitive, it’s worth considering this intuition in more detail. Inertia, Newton tells us, “is the power of resisting by which every body, so far as it is able, preserves in its state either of resting or of moving uniformly straight forward.”1 Applying this to the notion of potential we’ve been discussing, we may wonder why a potential needs to be continually actualized by some actuality in order to stay actualized. Making this a bit more precise, consider the following two conditions:

  1. At time t, potential P exists.
  2. At some earlier time t* < t, P was actualized by something else.

A little reflection will make clear that these are not sufficient to account for P’s being actualized at time t. If I was holding a book above the ground in order to actualize its potential to be a meter above the ground at time t*, but have since let it go, then by time t the book would be falling to the ground, and therefore the potential would no longer be actualized. Of course, this occurs because of the gravitational force applied by the earth on the book, and realizing this we might add a third condition to the two above:

  1. At time t, potential P exists.
  2. At some earlier time t* < t, P was actualized by something else.
  3. Between t* and t, nothing actualizes a potential P* that is incompatible with P.

But while this addresses our previous example, this is still insufficient. An example that illustrates this from more recent physics is radioactive decay, wherein an unstable atom will spontaneously emit particles of its own accord, and thereby unactualize certain potentials within itself. More generally, any non-equilibrium state of a system will lead that system to change the set of potentials that are actualized within it as it tends toward an equilibrium state. Both of these involve potentials which are actualized in a way that is inherently temporary when left alone. Once we realize that such “transiently actualized potentials” exist, we recognize this behavior in everyday things around us without needing to defer to such complicated examples. For instance, the clicking of my fingers is an actualization of a potential that inherently becomes unactualized almost immediately. And a burning fire tends to go out as it uses up the combustible molecules in the wood.

You’ll notice, however, that none of these examples mention the motion of physical objects in straight lines. And that’s no coincidence, since inertia applies in those cases. The point of these examples is not to somehow disprove inertia, but rather to show the failure of a certain approach to questions of actuality and potentiality. Inertia is a very specific physical principle, which cannot be applied to such a general metaphysical notion as potentiality. Rather than trying to understand actuality and potentiality in terms of inertia, therefore, we should instead try to understand inertia in terms of actuality and potentiality. In doing so we will see how inertia is in no way at odds with our earlier conclusion about the actualization of potentials.

With this reorientation in hand, we can ask: what needs to be added to our three conditions in order to properly characterize inertial behaviors? We’ve said that a potential of itself is indifferent to what is actual. Since the continued actualization of a potential is not indifferent to what is actual, it follows that we should be looking for an actuality. And not just any actuality, but an actuality that is somehow ordered to maintaining the actualization of P:

  1. At time t, potential P exists.
  2. At some earlier time t* < t, P was actualized by something else.
  3. Between t* and t, nothing actualizes a potential P* that is incompatible with P.
  4. Since t* until t, some actuality A maintains the actualization of P.

Importantly, this actuality doesn’t need to be something external to the thing whose potential we’re considering — as Newton said, inertia is in some sense the power of a body — it just won’t be the potential itself. We could call this actuality the “inertial actuality,” since it is the source of the inertial behavior. At the level of generality that we’re considering it here, inertial behaviors and actualities are not restricted to physical inertia. Just as there are many different ways that actuality and potentiality come to be in the world, so too there may be many different kinds of inertia. Nevertheless, we can characterize inertia in general in terms of another category, and use physical inertia as a paradigm case.

The category I have in mind is the Aristotelian form. Form and matter are two mutually intelligible categories, at least when it comes to material things. Generally speaking, matter is a substratum of some kind that is indeterminate of itself, and form is the determination of that substratum to one of the alternatives.2 In the case of physical things, matter is that which underlies all physical reality, and form is that which determines what kind of thing each physical thing is. It’s because matter is determined in a particular way that some physical things are trees, others are rocks, others sub-atomic particles, and so on. The indeterminacy of matter corresponds to its potential of being in different ways, and the determination of this matter by the form is an actualization of one of those potentials. Thus, a form, as an actuality, is a ready candidate for being an inertial actuality.

And indeed, the Aristotelian notion of form does well in accounting for physical inertia, both in terms of how Newton originally conceived of it (his rejection of the notion notwithstanding), and in terms of how physicists have conceived of it since. On this account, inertia as a feature common to forms of all physical things, as something that flows from the determination of matter regardless of the form that is doing the determination. Not only is form an actuality internal to a thing, but it is also common for forms to only partially determine matter, leaving it up to further forms to complete them. For instance, the primary (or substantial) form of a squirrel determines its underlying matter to be ordered in a certain kind of activity of life, but is indifferent to the exact details of that life, such as location, size, strength, and so on. These variables are provided by the primary form, to be fixed by secondary (or accidental) forms that augment the exact shape of that life at different times. Likewise, in the case of inertia, the primary form of any physical thing orders that thing in such a way as it maintains its rectilinear motion, but is indifferent as to exactly which inertial frame it’s in. This variable is provided by the primary form, but is fixed by other causes, which thereby impart the relevant secondary form of the precise rectilinear motion to be maintained.

Now, Aristotle knew that the primary forms of natural things would move them in the absence of some countervailing influence. His mistake was his particular conception of this motion: he thought that it was always ordered toward a specific place or in a specific direction, with light things inherently moving upward and heavy things inherently moving downward. The inadequacy of this particular conception of physical motion notwithstanding, his broader theory of forms is still a valuable tool in accounting for modern conceptions of physical inertia.

So, the primary forms of physical things are the inertial actualities that account for their physical inertial behavior. Moreover, we can flesh out the picture as follows. Every primary form of a physical thing will be the actualization of a potential in the matter underlying that physical thing. Applying our argument from the indifference of potentials, it follows that this form is actualized by some other actuality. And since this form is what grounds the existence of the physical thing in question, this other actuality must be the actuality of something else. But it’s not as though this cause will be some other perpetually-conjoined physical thing, since physical things only act on other pre-existing physical things, while this cause is sustaining the existence of the physical thing in question from moment to moment. The primary form a physical thing, then, is a metaphysical “threshold” of sorts, beyond which we move from physical actualities to non-physical actualities.

This leads us to another sort of inertia that is discussed in metaphysics, namely existential inertia. This refers to the inherent tendency of things to stay in existence in the absence of countervailing influences. In the terms of what we’ve been discussing, it’s the notion that once a physical thing has been brought into existence, we don’t need a “something else” to keep it in existence. There are, I think, two motivations that might be given for existential inertia, each problematic in their own way. First, we could motivate it by analogy to physical inertia. The problem with this, as we’ve seen above, is that such “inertial explanations” require an inertial actuality, and since we’re considering the actualization of a thing’s potential for existence, this further actuality cannot be something internal to that thing. Second, we could motivate it by generalizing the observation that the things we experience don’t require continually conjoined causes to keep them in existence. The problem with this is that it’s looking for the wrong sort of cause, not realizing that in talking about the cause of a thing’s continual being we’ve crossed that metaphysical threshold we just mentioned. We are not saying that this cause somehow acts on that thing, as if to presume that the thing somehow pre-exists the acting, but rather that by acting the cause actualizes the potential whereby the thing has existence from moment to moment in the first place. If this cause acts “on” anything, it’s on the thing’s constitutive principles, such as its form and matter, not the thing itself.

There is, however, something to be said for a qualified version of existential inertia: insofar as something is determined to exist, it is determined to continue existing. Thus, many things tend to preserve themselves in existence until they’re destroyed, or the underlying resources that they depend on run out. But this flows from the primary form of such things, and so as before if this form is an actualized potential then it will need a cause.

Further reading

If you’re interested in reading more about inertia and related topics, I can recommend Sean Collins’s paper “Animals, Inertia, and the Concept of Force” (or his related blogpost Animals, Inertia, and Projectile motion), Thomas McLaughlin’s paper “Nature and Inertia” (JSTOR), the exchange between Edward Feser and Michael Rota in the Proceedings of the Society for Medieval Logic and Metaphysics (vol 10), and Feser’s blogpost Oerter on inertial motion and angels.


  1. Isaac Newton, The Principia (def 3), cited in Thomas McLaughlin, “Nature and Inertia.”
  2. See my discussion on the threefold whole for an extensive discussion on form and matter. To see that these general definitions extend beyond simple physical objects, consider the following examples. The matter of a wooden table is the underlying wood, since it is indeterminate between various ways of being used, and the form of the table is how the wood is cut up into pieces and structured together in a particular way. The matter of my action of punching something is the motion of my arm, since the same motion is present in different actions, and the form of my action is my intention to damage something. The matter of a person educated in some field is the person considered without regard to their education, and the form is their educatedness or uneducatedness.

8 thoughts on “Potentiality and inertia

  1. Hello.
    I was thinking maybe some thoughts here can also help with another issue with potentiality, there seems the be a counter-example with thinking of actualization of potential as necessary condition for causation.Consider the case of simultaneous causation.If some A eternally simultaneously causes B then at least some sort of causation in the case isn’t the case of actualization of some unactualized potential. of course we can still say B is actually being sustained by A but B never went from being properly non-actual to actual.

  2. Hey Nasir, sorry for taking so long to reply!

    Your counter-example seems to assume that every actualization of a potential involves a previously existing unactualized potential becoming actualized, but this is not the case. A potential is also actualized when it is sustained in actuality. Think, for instance, of my phone sitting on the table, a meter above the ground. In this case, there are two different actualizations of the same potential:

    1. When I moved my phone there I actualized a previously unactualized potential for it to be a meter above the ground.
    2. When the table holds the phone above the ground, it continually actualized the phone’s potential to be a meter above the ground, and as soon the table stops doing this the phone falls to the ground.

    The first of these involves motion, which is why Aristotle defines motion not simply as the actualization of a potential, but rather as the actualization of a potential *insofar as it is potential*. The second of these involves the sustaining of a potential in actuality, which we could call the actualization of a potential *insofar as it is actual*. (I discuss this briefly in an older post here: https://thinkingthoughtout.com/2015/08/31/actualisation-of-potentiality-as-such/ Although, be warned, I wasn’t being as precise there as I am in this post.)

    So, your example isn’t a counter-example to the claim that the actualization of a potential is a necessary condition for causation. Rather, it is a counter-example to the claim that *motion* (the actualization of a potential insofar as it is potential) is a necessary condition for causation. And this is quite right, but I don’t know of anyone who claims that!

  3. Hi Again
    Thank you for the response.It seems there really is something to your response but I find problematic sort of of cases in which simultaneous cause and effect are past eternal.In THAT case although we can still say that the cause is keeping the potential of the effect actual.We won’t be able to say that cause has actualized the potential for the effect to exist in the first place.But we can still understand relation in causal term like we can use the language like Had cause not existed the effect wouldn’t have either but we can’t say effects existence in the first place is a case of actualization of potential in any sense. So it seems we have a certain causal claim which isn’t relation to actualization of potentiality.

  4. Hey Nasir, apologies again for taking so long to reply.

    I understand your concern as follows. You consider the following potential causal facts:

    1. Some effect E is brought into existence by some cause C.
    2. Some effect E is sustained in existence by some cause C.
    3. If C had not existed, then neither would its effect E.

    And your concern is that some of these cannot be accounted for in terms of actualization of potentials. In your first comment you said only (1) could be accounted for, but since my response you tentatively accept that (2) might also be accounted for (as the actualization of a potential insofar as it is actual). But what about (3)?

    Now, assuming I’ve got your concern right, I think it helps to realize that (3) is not a new fact over and above (1) or (2). In fact, (3) is ambiguous between the following:

    3.1. If C had not existed, then E would not have come into existence.
    3.2. If C had not existed, then E would not have been sustained in existence.

    As you can see, (3.1) parallels (1), while (3.2) parallels (2). And the reason for believing (3.1) or (3.2) will be parasitic on our reasons for believing (1) or (2) respectively (together, perhaps, with some additional belief in essentiality of origins or some claim like that — but we can ignore that for the purposes of the example).

    Now, in your simultaneous past eternal example, neither (1) nor (3.1) will be true, since the effect in question never began to exist. (2) will be true, and analyzable in terms of the actualization of a potential insofar as it is actual, and (3.2) will be true for much the same reason: if C had not existed then it would not have been around to continually actualize the potential of E’s existence.

    So, it seems to me that there is no causal fact that cannot be analyzed in terms of the actualization of potentials. We just need to keep in mind the distinction between actualization of a potential that was previously unactualized (ie. insofar as it is potential) or the continued actualization of a potential (ie. insofar as it is actual). This is effectively what I was getting at in the third paragraph of the post, where I said the following:

    “I also retain the potential to sit down while I’m actually sitting down, since I can’t be actualizing a potential I don’t have. This can be missed because we sometimes speak of the potential to *become* actual in some way rather than the potential to *be* actual in some way, even though the latter is more fundamental…”

  5. Thanks for the reply once again, I would like to discuss some points further. Please let me know if you want me to stop as I know combox discussions can become tedious. I followed your blog through your facebook groups posts, your writings are very good.

    “(3) is ambiguous between the following:

    3.1. If C had not existed, then E would not have come into existence.
    3.2. If C had not existed, then E would not have been sustained in existence.”

    That is a good point, I hadn’t considered that, but on the other hand it seems to me that 3 might not be reduced to claims like these. 3 could be simply true, period. But if it can be , then your points do follow I think.

    Other thing I’d like to discuss is, if we agree with you does that mean that in any cosmological argument that uses act/potency distinction we would need to show that creation began to exist? as we wouldn’t be able to say that is was actualized in a particular sense if its past eternal? This might be related to how 3 is analyzed.

  6. Hey Nasir,

    I’m happy to continue in the combox for a bit, I just can’t guarantee timely responses 😛

    “… but on the other hand it seems to me that 3 might not be reduced to claims like these.”

    This isn’t so much a reduction as it is a disambiguation. If you know of another sense which the original sentence may have, then we can consider it to see if it reveals anything interesting over and above what’s been said. I’m not sure what that other sense could be, however, if the causation of existence is exhausted by bringing something into existence and sustaining something in existence.

    “Other thing I’d like to discuss is, if we agree with you does that mean that in any cosmological argument that uses act/potency distinction we would need to show that creation *began to exist*? as we wouldn’t be able to say that is was actualized in a particular sense if its past eternal? This might be related to how 3 is analyzed.”

    I don’t think so, no. The burden of my post is any actualized potential depends on another actuality *for its continued actualization*, regardless of whether that thing ever began to exist or not. This would be true of past eternal actualized potentials as well. We’re not looking for the first time the potential was actualized, but the actuality that sustains its actualization for every moment it is actual.

  7. Roland.
    I meant to say that 3 might be true in a more basic way. Some eternally existing effect depends on its cause simpliciter, if the effect hadn’t existed so the cause wouldn’t have either. We can put this in terms of possible worlds language if that makes sense, the effect only exists where the cause exists.

    We’re not looking for the first time the potential was actualized, but the actuality that sustains its actualization for every moment it is actual.

    Right, but then wouldn’t it follow that such argument would only prove a sustainer of the creation, not the creator?
    Speaking as someone who accepts the conclusion of the argument, not sure if it would be a huge problem but it might create some difficulties.

  8. Nasir,

    “I meant to say that 3 might be true in a more basic way. Some eternally existing effect depends on its cause simpliciter, if the effect hadn’t existed so the cause wouldn’t have either.”

    If I’m understanding correctly, the suggestion is that the counterfactual fact might be basic, and therefore give us a third sense of “depends on”? One could certainly posit this, I’ll give you that, but they would still need to tell us why this counterfactual is not reducible to facts about the things themselves. (Otherwise they would be simply be asserting the bruteness of a fact in the face of a reduction of that fact, which is far from reasonable.) Perhaps this would look like some neo-Humean account of possible worlds, or something else. Certainly such proposals exist and are discussed, but I struggle to see the motivation for them for two reasons. First, I can’t help but get the sense that they are proposed to solve problems that wouldn’t exist if we had started with a more robust account of reality, such as the Aristotelian does. (See Pruss’s Actuality, Possibility, and Worlds.) Second, the contemporary discussion of finks, masks, etc. suggests to me that the counterfactual accounts of causation (or ontological dependence) are not nuanced enough to be general in the way that they need. (See section 1.2.2 of Feser’s Scholastic Metaphysics.)

    “Right, but then wouldn’t it follow that such argument would only prove a sustainer of the creation, not the creator?”

    Yes, if by creator you mean “that being which brought everything else into existence.” If you want to establish the beginning of everything you would need a different argument. Aquinas famously thought that the existence of God could be established via argument, but not the beginning of the universe: he thought that scripture taught us that the universe did in fact begin to exist, but that we could not establish this via reason. And the argument we gesture toward in this post is pretty much Aquinas’s Second Way, framed in terms of actuality and potentiality. Of course, God being the creator (in the above sense) is perfectly consistent with him being the sustainer, it’s just that different arguments seek to establish different things. We could supplement this argument with some Kalam-like argument if we also wanted to. I very much like the sort of arguments discussed by Aristotle and Aquinas, because they don’t depend as much on discussions about contemporary cosmogony, and in the end they enable us to reason out a lot about the first cause, since they are framed in very general terms like actuality and potentiality. (Indeed, this is what Aquinas does for much of questions 3 to 26 in his Summa Theologica, http://www.newadvent.org/summa/1.htm)

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