Hylomorphism vs. Substance Dualism: What's the Difference, Really?

It all depends on how one defines substance dualism...

Despite the loud, boisterous, and occasionally Latin-filled protestations of many Aristotelians and Thomists—aka the venerable, bearded, cigar-chomping (and, let’s be honest, sometimes painfully snobby) hylomorphs, of which I proudly count myself—it’s not entirely or automatically incorrect to consider hylomorphism a form of substance dualism, provided we’re clear about what we mean by each respective theory.¹

For example, if by substance dualism one simply means that the human person is composed of a substantial immaterial soul and a physical body, then hylomorphism certainly fits that description. However, if by substance dualism one means the (somewhat cartoonish) “ghost in a machine” view—where two wholly independent and complete substances (one immaterial, one material) are somehow joined together and interacting—then hylomorphism is decidedly not a form of substance dualism.²

Whether or not any serious philosopher of mind actually holds the latter view of substance dualism is a question I’ll set aside for now.³ My focus here is to use the "ghost in the machine" version of substance dualism as a comparison point for hylomorphism. I believe this exercise will be illuminating, if only to better understand why so many Aristotelians and Thomists vehemently reject being associated with such a picture of the human person.

So, right. Hylomorphism—Aristotle’s theory that all material beings are composite entities made up of two fundamental constituents or co-principles, namely matter and form (the term itself essentially translates to "matter-form-ism")—is, to reiterate, most definitely not a form of “ghost in the machine” substance dualism (GITMSD, for short?). Why? For the (I think supremely) obvious reason that hylomorphism concerns what constitutes a single material substance. (A substance, traditionally, is a single, “tightly” unified entity.)

GITMSD, on the other hand—as its name implies—posits two substances, typically an immaterial substance interacting with a material substance. For the substance dualist of this sort, there are two distinct substantial entities at play (soul or “ghost” and body or “machine”, respectively ; ). In contrast, for the hylomorphist, there is only one substantial entity, composed of two distinct but incomplete constitutive elements: form and matter. (For living beings, their form just is their soul, which organizes and configures a particular parcel of matter into a unified, functioning body.)

As the language above suggests, one way to grasp this difference is by considering the notion of completeness. For the GITMSD, the interacting entities—soul and body—are regarded as complete substances in and of themselves, somehow linked together. But this is definitely not the case with hylomorphism. For the Aristotelian, form is the configuring principle (that which configures), and matter is the substrate (that which is configured). Again, these are not independent substances but rather two “causes” (or principles, if you prefer) that account for different aspects of a single being. (Side note: While this is not typical in Aristotelian literature, I’ve suggested in various places that it can be helpful to think of these co-principles along Fregean lines of incomplete entities. Each requires the other for supplementation or completion to form a whole, unified substance. Form is incomplete with respect to individuation, while matter is incomplete with respect to configuration. Together—and only together—do they constitute an actual, complete substantial entity, and neither is generally capable of independent existence.⁴)

For those familiar with traditional hylomorphism, it’s often emphasized that its primary motivations have little to do with contemporary debates surrounding consciousness or the mind-body problem. Instead, hylomorphism is rooted in much more fundamental—and, to my mind, more interesting—issues within the philosophy of nature. Chief among these are the problems of unity in diversity (the classic “one and the many”) and change.

For example—as the traditional maxim holds—nothing can change unless something remains the same. To address this, Aristotle provides a constituent ontology, positing that familiar material particulars are composed of various metaphysical “parts,” such as matter and form (both substantial and accidental). These constitutive elements, Aristotelians argue, are necessary conditions for explaining how change is possible. They allow us to understand how a single material object can undergo various contingent modifications—like going from sitting to standing or changing in color or temperature (by acquiring or losing accidental forms)—while remaining fundamentally the same substantial thing. Moreover, this theory provides an account of how substances are generated and corrupted—when prime matter is configured by different substantial forms. (For example, a tree might be reduced to ash after burning, with its underlying matter taking on a new substantial form.)

What’s interesting, of course, is that while Aristotle’s hylomorphism wasn’t devised with modern philosophical concerns in mind, it happens to offer significant resources for resolving contemporary issues—particularly in the philosophy of mind, say, with respect to the mind-body problem or what have you. This is why many sharp philosophers (strong and handsome, too, if we are thinking of Jim Madden) today adopt hylomorphism to address just these sorts of problems. Nevertheless, it is indeed important to emphasize that the primary motivations for hylomorphism are significantly “upstream” of these modern debates about consciousness or mind-body interaction. Its original purpose was to resolve metaphysical puzzles about change, unity, and diversity. Only much later do we see its application to contemporary issues, revealing its—impressive, I’d say—versatility.

Now, in case it isn’t obvious, to suggest that hylomorphism is a form of GITMSD completely undermines the whole point of why the theory was proposed in the first place—namely, to explain how a single substance can be generated, corrupted, or undergo various modifications. If you start saying there are two substances at play (form and matter?), you’ve made the theory totally useless for the job it was meant to do. So, while one might perhaps still insist that hylomorphism is a form of dualism, perhaps even a (properly qualified) form of substance dualism, it is just totally wrong to say it is a form of “ghost in the machine” substance dualism.

Final point—this one something more of an historical note and relating to the so-called interaction problem. I have—at least online, anyway—heard proponents of hylomorphism claim that because we’re talking about just one substance (which is correct), the interaction problem doesn’t arise for hylomorphism. But that’s not quite right. The interaction problem—that is, how an immaterial entity can “interact” with a material body—isn’t really about whether there’s one or more substances at play. Rather, it arises from a prior commitment in the philosophy of nature, specifically a mechanical or mechanistic view that reduces causation to crude efficient causality: stuff (with extension) bumping into other stuff.

OK, so, there’s a lot to say about why this mechanistic view is just awful—not least because it doesn’t even fit with much of contemporary science—but for now, it should be obvious why someone who thinks interaction requires literal “bumping” might see a problem when positing immaterial entities. The solution, however, isn’t necessarily to abandon substance dualism, but to toss out the mechanistic worldview that created this crude division of mind and world in the first place.

How so? Well, the (super-simplified) story goes like this: the “ghost in the machine” paradigm was effectively forced into existence by the rise of a quantitative, mechanistic, and reductive philosophy of nature that displaced Aristotelianism. In this post-Aristotelian worldview, the physical world was reduced to pure quantity—things like mass, motion, and extension—with no obvious room for features like consciousness, intentionality, etc. As a result, the "mind" or "soul" was imagined as something radically separate from the physical, awkwardly inserted into this world of quantity, and then expected to somehow interact with it—mysteriously, of course. (Alternatively, some thinkers, rather than embrace substance dualism, opt simply for eliminativism, outright denying the existence of mind or soul altogether, as commonly understood. Indeed, such a background philosophy of nature seems to leave only these two options, forcing a move in one of these directions.)

The Aristotelian hylomorphist, by contrast, doesn’t lose sleep over the interaction problem because they reject the mechanistic assumptions outright. Of course Aristotelians affirm efficient causality—but in a far richer and non-reductive sense than mechanists do. More importantly, Aristotelians recognize other modes of causation as well: formal, material, and final causality. And once these broader causal categories are in place, the interaction problem dissolves—or, at the very least, anyone raising it ends up begging a much larger question against the Aristotelian position. (More accurately, the Aristotelian worldview avoids generating the problem altogether, since the so-called interaction problem only arises when one confines themselves to an overly narrow conception of efficient causality within a reductive and impoverished philosophy of nature.)

Again, why is this? Because, for the Aristotelian, the soul doesn’t “interact” with the body any more than the form of a cat “interacts” with the cat.⁵ To think otherwise is a significant conceptual error; for sure, this kind of misunderstanding treats the constituent elements—form and matter—as if they were two distinct, substantial entities interacting via efficient causality. But that is precisely what the Aristotelian denies.

Anyway, once all this is understood—which, for many, might require serious conceptual reworking, given how the mechanistic view of nature “holds us captive”—it becomes clear why the so-called interaction problem is, for hylomorphism, a complete non-issue. The soul and body, as form and matter, are not separate, efficiently interacting substances but constitutive principles of a single, unified entity. (Of course, whether some further cause is required to account for the unity of these constituents—especially if one argues they are not united purely in virtue of what they are, as I would—is a fascinating question, but that’s a topic for another day.) The problem simply doesn’t arise because the Aristotelian worldview avoids imposing such a crude division of reality in the first place. Simple as that.

1. It really all comes down to this: if by substance dualism one means a single substance composed of two really distinct constitutive elements, this is not incompatible with hylomorphism. However, if by substance dualism one means two entirely distinct substances, then that is definitely incompatible with hylomorphism.

2. This view, as Edward Feser tells us, is just one part of the broad Cartesian view of the world. That is, that the self is really a wholly incorporate thinking substance, where the body is part of the “external” world. The other parts of Cartesianism include a commitment to the theory of innate ideas and indirect realism. For more, see Immortal Souls, chapter 7.

3. For example, J.P. Moreland, one of the more prominent defenders of substance dualism, definitely does not hold this cartoonish version—in fact, he actually defends a broadly Thomistic/hylomorphic view. See The Substance of Consciousness.

4. I say generally because this matter is complicated by the existence of immaterial entities and post-mortem survival issues. For immaterial entities, their form is such that no matter is required or even possible—however, because they have no material principle of individuation, such immaterial entities are truly one of a kind; ST. Michael just is St. Michael-ness. And because human beings, by nature, have an immaterial aspect (related to intellect) this part survives bodily corruption; however, what is left is really an incomplete person.

5. Example borrowed, with slight modification, from Feser’s Immortal Souls.