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.

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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.↩