Category Archives: Material Substance

Hylomorphism and Modern Physics

Question: “Thomistic hylomorphism is often ‘sold’ to contemporary audiences as the best solution to contemporary problems in the philosophy of mind. But is Thomistic hylomorphism compatible with contemporary physics? For if not, then it’s a non-starter.”

Answer by Andrew J. Jaeger and James D. Madden

(Benedictine College Department of Philosophy)

We have been asked to consider the plausibility of a hylomorphic philosophy of mind in light of the results of modern physics. We begin by applauding the insight behind this question. Many contemporary presentations of hylomorphism emphasize the theory’s supposed virtues in the philosophy of mind, but we worry that in many cases this sort of presentation puts hylomorphism to uses for which it was never designed. For this reason hylomorphism frequently suffers charges of obscurantism and incoherence, even from contemporary philosophers one might otherwise expect to be sympathetic to the conclusions of a broadly Thomistic philosophy. One of us has touted hylomorphism as a way of dissolving the vexations of contemporary philosophy of mind in great deal (see Madden, James D., Mind, Matter, and Nature: A Thomistic Proposal for the Philosophy of Mind, CUA Press: 2013), but even there it is emphasized that hylomorphism should first and foremost be taken as a philosophy of nature, and only secondarily should it be applied to other philosophical arenas. We agree that, if indeed hylomorphism were unable to accommodate the well-verified results of modern physics, its plausibility for the philosophy of mind would be greatly diminished, because its plausibility as a philosophy of nature would be greatly diminished.

In what follows, we will argue that a common version of hylomorphism is not in fact in tension with the results of modern physics. Though we will have little to say about the philosophy of mind here, we will begin by presenting a version of hylomorphism we believe alleviates the stresses imposed by the modern mind-body problem (see Mind, Matter, and Nature for the details). We then consider the results of modern physics most likely to be taken at odds with hylomorphism. In each case we argue that there is no, to borrow a phrase from Alvin Plantinga, “deep conflict” between hylomorphism, as we have construed it, and modern physical theories.

Before proceeding, four clarifications are in order. First, we make no claim that the version of hylomorphism presented below represents in detail that defended by any particular historical individual. Certainly, we believe that our version of hylomorphism is something akin to what St. Thomas was actually up to, but our intention is produce a “Thomistic hylomorphism,” as opposed to “Thomas’s hylomorphism.” We will leave questions of historical accuracy for another day. Second, by “hylomorphism” we do not have in mind theses defended in classical and medieval Aristotelian science regarding projectile motion, celestial bodies, the nature of time and space, etc.   The degree to which these theses can be divorced from the hylomorphism also held by their historical proponents is likely a matter of some controversy, but for now, however, we leave those questions aside. Third, we will not attempt to demonstrate or otherwise defend this version of hylomorphism on philosophical ground, though we are confident such a justification is available (see Mind, Matter, and Nature, along with Jaeger’s “Back to the Primitive: From Substantial Capacities to Prime Matter,” American Catholic Philosophical Quarterly, 2014, vol. 88, no.3, pp. 381-395). Rather, we will simply present the essentials of hylomorphism while discussing some of its virtues. Finally, when we speak of “modern physical theories” we have in mind primarily the fundamentals of classical physics (viz. the Theory of Relativity) and quantum mechanics. Granted, the really interesting stuff comes only after one has mastered the nuances of such theories (and their various interpretations), but this is something we cannot hope to establish here. Therefore, we will content ourselves with investigating the relationship between the fundamentals of such physical theories and hylomorphism.


We define hylomorphism as the conjunction of the two following theses:

Metaphysical Composition Thesis (MCT): material substances are compounds of certain metaphysical parts, viz. matter and form.

Substantial Simplicity Thesis (SST): material substances do not have material substances as proper component parts.

There are certainly influential versions of hylomorphism whose proponents happily deny both MCT and SST, but these are principles affirmed generally by Thomists. As for MCT keep in mind that the hylomorphist of the stripe we have in mind sees this doctrine as first and foremost an attempt to answer the classical problem of change. By “matter” the hylomorphist simply means the principle of potency that pre-exists and survives a change; whereas by “form” the hylomorphist means the principle of actuality that comes to be as a result of a change. Thus, anything subject to change is – in the sense just described – a compound of matter and form. It is crucial for the overall plausibility of hylomorphism to emphasize that matter and form are first and foremost functionally defined (which is not to say that hylomorphists are functionalists in the philosophy of mind!). To reiterate, “matter” refers to whatever does the work of providing the potency and continuity in a change, and “form” refers to whatever does the work of providing the actuality of the change (see Brower, Aquinas’s Ontology of the Material World, Oxford: 2014, pp. 66-9, for a recent detailed defense of defining matter and form functionally in the context of change). Notice that whatever does the work of potency for a change cannot be essentially contrary to the form that comes to be, otherwise the change would be impossible. If some matter were essentially non-F, then it could not provide the potency and continuity for becoming F.   In a case of accidental change, i.e. the coming to be of a substance with a non-essential attribute, there is seemingly no difficulty in this vicinity. Since Smitty is not essentially short, there is no problem posed by Smitty being the underlying matter in a case of his becoming the contrary of short, i.e., Smitty may gain the accidental form of being tall because the matter of such a change is neither essentially short nor essentially tall.

Substantial change, the coming to be of a substance with an essential attribute, however, is a bit more difficult. Smitty is in some sense subject to substantial change, as his mortality obviously attests. Moreover, whatever composes Smitty materially could compose essentially different substances, e.g., Smitty’s matter might have composed several squirrels or a small pony. Nevertheless, Smitty cannot be the matter of such changes, because Smitty is essentially not a squirrel and essentially not a pony. The problem lies in the fact that anything possessing an essential attribute cannot be characterized by a contrary essential attribute. Thus, the principle of potency for a substantial change cannot itself possess any essential attribute. Since every natural substance is subject to coming to be and passing away, all such substances are not only composed of substantial forms, but also prime matter, an underlying principle of potency that possesses no essential attribute intrinsically.[1]

As for SST, keep in mind that Smitty is not only subject to change by alternation of contrary attributes, but also division, e.g., Smitty might, sadly, lose his pinky finger in an unfortunate encounter with a can opener. Attempts to solve the problems of division have spawned a proliferation of co-located or overlapping entities, e.g., Smitty-minus-pinky or even a distinct Smitty-minus for each quark currently in his composition, simultaneously occupying the space roughly outlined by Smitty’s (or Smitties’?) skin. Hylomorphists, or at least those who are more traditionally Thomistic, prefer to avoid this multiplication of entities by arguing that parts that can be divided from a substance are not substantially present (i.e., present as substances) but are rather virtually present in Smitty (or nominally present, as a Patrick Toner puts in “Emergent Substance,” Philosophical Studies, December 2008, vol. 141, no. 3, pp. 281-297). The point here is that Smitty’s parts, including his hands, feet, organs, etc. right down to his subatomic particles, do not exist as individuated substances while they compose Smitty. Rather, the parts of a substance, however fundamental, only exist through the substances. Some hylomorphists who accept PT claim that such parts can be retrieved as individuated substances, though while “they” are in the composition of a material substance, these parts exist only as powers or potencies of such a substance. Other proponents of SST emphasize that the subatomic particles that are virtually present in a natural substance belong to a different substantial kind from those that are substantially present. So, in a real sense, atoms stop existing when they become parts of a substance. On the view, it is at least an open question whether those same atoms can be retrieved or not, for such “retrieval” would amount to a substantial change (see Toner “St. Thomas Aquinas on Mixture and the Gappy Existence of the Elements,European Journal for Philosophy of Religion 3 (1):243 – 248 for an argument in favor of retrieval). We grant, of course, that the doctrine of virtual presence strikes the contemporary ear with scandal, but this doctrine is no less scandalous than the notion that Smitty is actually a whirling conglomeration of billions of overlapping human organisms, something that is claimed by many contemporary metaphysicians without blushing. To mitigate this appearance of scandal, keep in mind that SST doesn’t entail that material substances lack parts entirely, just that anything that is a part of a material substance can’t be a material substance and so must be ontologically posterior to the whole of which it is a part.


Explaining what relationship holds (more importantly, ought to hold) between metaphysics and physics is a significantly complicated affair. To do it justice, one ought to weigh in on various debates in epistemology and the philosophy of science; however, we will not do so here. Our current goal is not a defense of any one account of the relationship between the two, but rather a discussion of the various ‘live options’ on the table for hylomorphists to consider. With that said, and in the interest of space, we will not consider objections to what we say below – we offer something of an abbreviated menu rather than the real four-course meal.

Before turning to special relativity (SR) and general relativity (GR), let’s quickly consider the principle that underlies them both – a principle dating back several hundred years. This principle – which is somewhat misnamed – is often stated as follows.

The Principle of Relativity: the physical laws hold in all inertial reference frames.

What implications does the principle of relativity give rise to? Well, this seems to imply that one cannot determine by physical experimentation from within a single inertial reference frame, which reference frame one is in. To borrow an example from Peter Kosso (see Appearance and Reality: An Introduction to the Philosophy of Physics, OUP: 1998, pp. 33),

When you wake up on the train and the window shades are closed, you cannot tell if the train is moving or not. Similarly, when the adjacent train at the station begins to move relative to you, it is hard to tell who is moving away from the platform, you or they.

This – rather old – phenomenon has given rise to several concerns among Thomists regarding the analysis of local motion in hylomorphic terms (i.e., in terms of matter/potency and form/actuality). Whether one is moving might then turn out to be a matter as to which reference frame(s) an object is being observed from. But, this would seem to imply that whether a potency is actualized is a relative affair (i.e., an issue relative to a reference frame). Fearing relativism (of the bad variety), some hylomorphists have been averse to embracing the quite simple (and apparently obvious) principle of relativity at face value.

For various interesting reasons pertaining to the physical laws and the speed of light, the principle of relativity has given rise to a couple of theses that are integral to the theory of special relativity. The first thesis being that space (or location) turns out to a relative property. From one inertial reference frame, two events can be said to occur in the same location, but from a second inertial reference frame, the two events can be said to occur in different locations. So, “being-located-at” turns out to be a property relativized to an inertial reference frame. The second thesis holds that time is a relative property. Two events that occur at the same time from within one inertial reference frame occur are different times from a different reference frame. This is sometimes referred to as the “relativity of simultaneity.” That simultaneous events are only relative to some inertial frames but not to others. So, “occurs-at” is yet another property that turns out to be relative on the SR. From here, it seems that in order to describe the world and its events accurately, given these two consequences of SR, one must make reference to the world’s objects and events in (or at) particular inertial frames. One implication of this is that there is no true reference-frame neutral description of the world; how long something is (both spatially and temporally), when something occurs, where something is, are essentially relativized features of the world.

In light of this phenomnon, a hylomorphist might be tempted to skirt a realist interpretation here, allowing for a privileged reference frame that is explanatorily prior to the others. However, this will likely carry with it severe costs. Another option for hylomorphists is to hold that there are many more truths in the physical world than one had previously thought. Prior to Einstein, one would have thought that the explanation of Smitty’s location required no more than indicating his place within our reference frame. However, unbeknownst to us at the time, there are a multitude of other facts we never fathomed existed (and hence in need of explanation). After Einstein, when asked ‘Where is Smitty located?’ we’ve come to realize that that question has several answers rather than one. This certainly doesn’t entail to Smitty’s location is relative in the sense that it is a matter of human convention where he was located; SR does not entail a sort of subjectivism or idealism. There is an absolute fact of the matter where Smitty is located in reference frame K. It just so happens that in a different inertial frame, say K’, Smitty has a different location. To answer “Where is Smitty really?” one would have to describe his location in all reference frames where Smitty exists.[2] Each description of his location in a reference frame is an absolute truth. There is no threat of subjectivism here.

Lastly, just as some contemporary philosophers are inclined to speak of time-indexed properties (or time-indexed-instantiations of properties), a hylomorphist could easily speak of reference frame potencies and actualities. Rather than an object having a potential to be green, it would have the potential to be green-in-frame-K (or one might say it has-in-frame-K the potential to be green). This would allow for a material substance to actualize a potential in one frame, but not in another (alleviating what one might call “the problem of referential intrinsics”).[3] One can still account for change in hylomorphic terms while taking SR at face value. In short, within any given frame, SR does nothing to cast the basic hylomorphic account of change we have sketched above into doubt. All that is required is that the hylomorphist admit that there are more hylomorphic changes occurring than meets the eye, but that is no “deep conflict” with hylomorphism as such.

The general theory of relativity (GR) introduces questions deserving of more space and time than we can manage here (this pun is fully intended); however, a brief remark is in order. The general theory of relativity requires one to confront debates over the nature of spacetime, resulting in significant implications for the substantivalist vs. relationalist debate. It isn’t obvious whether a hylomorphist – in virtue of affirming MCT and SST – need take a stand on that debate, and we are inclined to think that no deep conflict arises between hylomorphism and either account of spacetime. One might find spacetime substantivalism inconsistent with hylomorphism for the following reason. It is typically held by hylomorphists (ultimately stemming from SST and a doctrine known as the unicity of substantial forms) that no two substances can be collocated. But, if spacetime is a substance, then it would seem to follow that no material object in spacetime (e.g., Smitty) would be a substance.[4]

Although substance monism (or more commonly known as priority monism) is one interpretation of substantivalism, there are others. One could regard spacetime as a substance, but only analogously to material substances. Spacetime and Smitty should be regarded as belonging to different modes of being. Therefore, what the substantivalist hylomorphist should say is that no two substances of the same mode of being can be collocated. Moreover, this is likely what classical hylomorphists intend to hold in the first place. General relativity involves much more than the debate over the nature of spacetime. All we have done in this paragraph is to open the door for ways of allowing a hylomorphist to begin to think about one of the common debates that has been affected by general relativity. We hope to examine the relationships between general relativity, the topology of spacetime and classical hylomorphism elsewhere in greater detail.

One might suppose that a further problem for the hylomorphist is posed by various well-known experiments, e.g., most famously the split screen set-up, that seemingly show things in “the quantum world” to be “in superpositional states, composites of incompatible attributes. The indeterminate quantum properties are made determinate by us in the act of observation, by the physical interaction with our big, classical bodies and machines with the quantum things” (Koso, Appearance and Reality, p. 153). That is, it appears that, for example, photons are intrinsically neither wave-like nor particulate, and their “collapse” into one such attribute occurs only after observation or incorporation into a larger, macro-determinate system. We agree that the empirical evidence for quantum indeterminacy is overwhelmingly strong, so we are prepared to follow its ultimate philosophical consequences come what may. At the very least we will grant the experimental results as settled science.

That being said, there is a good bit of controversy as to what the experiments actually show. Certain interpretations of quantum mechanics do claim that there is a metaphysical indeterminacy in the quantum world, e.g., the photon’s nature is in fact indeterminate, while other interpretations argue for more modest epistemic conclusions, e.g., the intrinsic nature of the photon is in principle beyond our ken, even if we can estimate its current state within a range of probabilities.   There are also issues as to what counts as “observation.” Using the split screen experiment again, in this case observation is made by applying a laser beam to a photon stream. As far as we know, it is an open question as to whether the laser beam in fact interacts causally with the photon stream in a way that explains the experimental results without the need to posit an actual indeterminacy in the quantum world, though such interaction is as yet unknown to us. We will not weigh-in on these issues, not only because they go beyond our meager scientific pay grade, but because our view is that quantum indeterminacy, even under a strongly metaphysical interpretation, should really be taken as good news for the overall plausibility of hylomorphism! Let’s stipulate then that, per the metaphysical interpretations of quantum indeterminacy, the intrinsic natures of the most fundamental physical elements are in-themselves in superpositional states with respect to certain incompatible attributes.   That is, quantum particles, when separated from broader determinate systems suffer certain indeterminacies.

To see why this stipulation poses no problem for the hylomorphist, begin by remembering that the hylomorphist argues that matter, when considered in precision from its compositional role in a determinate substance, is indeterminate among a certain range physical attributes, even otherwise essential attributes. The intrinsic indeterminacy of matter to contraries is what provides the potency for the sorts of change substances can undergo at various levels of composition, e.g., the indeterminacy of Smitty between running and walking is what makes it possible for him to pick up the pace, or the indeterminacy of certain amino acids between the different types of muscle tissue is what makes eating meat a means for muscle growth for various types of animals. Moreover, the broader the compositional role of a certain kind of matter, i.e., the more types of substances that it can compose, the greater its range of intrinsic indeterminacies, and ultimately the hylomorphist claims all material beings are the beneficiaries of the universal indeterminacy of prime matter. Thus, the hylomorphist is not at all surprised to find that as more fundamental types of matter are discovered so too are greater levels of intrinsic indeterminacy. Indeed he or she should expect as much. In other words, the “closer” we come to prime matter, the more we expect proximate matter to resemble prime matter’s indeterminacy.

Moreover, the fact that interaction with or incorporation into determinate macro-substances serves to move otherwise indeterminate quantum entities toward determinate attributes is likewise exactly what the hylomorphist would expect. For the hylomorphist, the substantial form does the determining work necessary for matter to realize definite, essential attributes. The more fundamental the elements (the greater their compositional potency), the “less” form there is to do this determining work. Thus, as fundamental particles are removed from their incorporation into determinate systems (or as we would put, as elements are removed from governing substantial forms), the hylomorphist would expect indeterminacies to abound. That is part of what hylomorphists have always meant by the slogan that the substantial form is the principle of actuality of a substance, and that matter exists (determinately) through the substantial form.

One last – albeit incredibly brief – point of comparison between hylomorphism and quantum theory is in order. Some metaphysicians (most notably Jonathan Schaffer) have made use of SST as providing the best means for explaining the phenomenon known as quantum entanglement. SST is more often than not regarded as the more contentious of the two theses we use to define hylomorphism, but the ability to explain such tricky phenomena would do much to help the hylomorphist motivate this controversial point. So, not only is hylomorphism compatible with quantum physics, it apparently garners rational support from it. The details are essential to evaluating the strength of the justification, but that story will have to wait for another day.

[1] Some have worried that to speak of prime matter as a type of being lacking essential attributes is to predicate of prime matter an essential attribute, viz. lacking essential attributes, and so have concluded that prime matter as pure potency is incoherent. However, this objection turns on the false assumption that all true predications refer to a form which needn’t be endorsed. See Brower, Aquinas’s Ontology of the Material World, for a helpful discussion and defense of this possibility.

[2] This allows for the possibility of there being reference frames where in Smitty isn’t located and so doesn’t exist. This is due to the fact that the light cones of objects in those reference frames are ‘outside’ of the Smitty’s light cones.

[3] How can one object have incompatible intrinsic properties in different reference frames; or even worse, how can an object exist in one inertial frame, but not in another? For a discussion of this problem in relation to the problem of existence and change, see Jaeger and Madden, “Existence and the Problem of Referential Intrinsics” (in progress).

[4] This view of spacetime substantivalism naturally gives rise to various forms of monism. Jonathan Schaffer has defended this version of monistic substantivialism at great length, see “Spacetime the one substance,” Philosophical Studies, 2008, vol. 145, no. 1: 131-148.