Quantum Field Theory on the Edge of Metaphysics - Part I
The Interaction Problem of Mental Causation
Recently, I had an interesting conversation with Tina Lee Forsee, during which we discussed several topics, including what is commonly referred to in the philosophy of mind as the 'interaction problem' or the 'problem of mental causation.' Since it has sparked a lively discussion about the meanings of 'physical' versus 'unphysical', I decided to address this question, which lies at the intersection of physics and metaphysics, from a more scientific perspective—specifically, through the lens of quantum mechanics (QM) and what quantum field theory (QFT) can reveal. This also provides an opportunity to clarify several misconceptions surrounding an old yet relevant philosophical debate.
What is this all about? Let's start with an introduction to the philosophical issue known as the 'interaction problem.'
Cartesian substance dualism is the philosophical view, proposed by René Descartes, that reality consists of two fundamentally different kinds of substances: the mind (a non-extended, thinking substance) and the body (an extended material substance).
Although contemporary philosophy has largely moved away from the original Cartesian dualism, non-physicalist theories remain prominent. Examples include dual aspect monism, panpsychism, cosmopsychism, idealism, substance monism, and various theistic or spiritualist accounts. Despite their differences, they share the idea that the mind is neither reducible to nor entirely dependent on the physical brain or body. Mental properties are understood as non-physical properties, with the body and mind regarded as distinct and separable entities or, at the very least, not reducible to exclusively physical properties.
On the other hand, these theories within the philosophy of mind share a common rejection by physicalist mind-body identity theories, which posit that physical processes or properties can fully explain mental phenomena. This is because asserting a fundamental distinction between the mental and the physical implicitly endorses some form of ‘interactionism’ or ‘interactive dualism’—the notion that the body and mind causally interact with one another despite being fundamentally different in nature. The same question applies not only to the mind but also to any entity or purportedly metaphysical substance, such as a soul, spirit, or 'immaterial' or 'unphysical' consciousness. This leads to one of the most enduring philosophical challenges, known as the ‘interaction problem’ or the ‘problem of mental causation,’ framed in 1643 by Princess Elisabeth of Bohemia, who presented a pointed challenge to Descartes regarding his Meditations on First Philosophy: “Given that the soul of a human being is only a thinking substance, how can it affect the bodily spirits in order to bring about voluntary actions?” (Shapiro, 2007).
In more modern terms, the issue can be expressed by asking how non-physical mental states—such as beliefs, desires, and intentions—can exert any causal influence in a physical world governed by physical laws. The dualist causal heterogeneity problem arises: If mental properties, states, or substances are radically heterogeneous from material objects and physical forces, they lack the commonality necessary for interaction. If the mind is immaterial, it remains unclear how it can causally interact with a material body without violating fundamental principles of physics and maintaining metaphysical coherence.
Another common objection to the concept of mental causation, dating back to Leibniz, is that such interactions would violate the principle of energy conservation. If physics encompasses all interactions among particles within the brain, then an immaterial mind influencing the brain's biochemistry would introduce a novel form of interaction at the microscale, ultimately violating the principle of energy conservation.
Put it bluntly, if in our brain exists an immaterial, unphysical entity that we call ‘mind’ (or ‘spirit,’ ‘soul’, ‘psyche,’ or whatever) how can this ghostly substance in our heads interact with our concrete and material world?
Of course, for materialists, this problem does not arise; there is no distinction between mind and brain. Our mental and psychological experiences are ultimately just biochemical reactions in our brain and body. Therefore, there is no need to explain any kind of interaction. In fact, the interaction problem serves as a philosophical challenge posed by physicalists to dualists: if you believe in the existence of a soul or an immaterial mind, you must explain how they could possibly interact.
Given that, despite centuries of debates, this issue remains unresolved and no consensus has been reached, the interaction problem is frequently invoked as a knockdown argument against any form of dualism and as a compelling rationale in support of physicalist mind-brain identity theories.
This three-part essay has two main aims. First, it seeks to highlight various semantic ambiguities related to the categories involved in the interaction problem, thus providing conceptual clarity in light of modern physics. Then I will emphasize the distinction between what we consider 'material' versus 'immaterial' and 'physical' versus 'unphysical,' clarify what an 'interaction' is according to the standard model of particle physics, and question what an extended versus non-extended 'substance' could possibly mean in the context of contemporary physics, particularly QFT. Finally I will examine a proposal for a principle of mental causation, with particular attention paid to the stochastic phenomena associated with quantum effects.
When Words Matter: Pursuing Conceptual Clarity in the Light of Contemporary Physics
1. What Does ‘Immaterial’ Mean?
From a physics perspective, the interaction problem is frequently articulated using ambiguous and potentially misleading terminology. For example, the inquiry regarding how an 'immaterial' mind can causally interact with a 'material' body implies a clear and well-defined distinction between the two or at least suggests that this distinction is self-evident and requires no further elucidation. Eventually, these opposing categories become conflated with the related notions of 'physical' versus 'unphysical.' However, at least from the perspective of the physical sciences, this conceptual ambiguity prevents a deeper understanding of the subject.
In physics, the term 'material' encompasses all entities composed of matter—specifically, those that possess mass. Mass serves as a measure of inertia, which is the property of a particle or body that enables it to resist changes in its state of motion. A massive particle can always be accelerated or decelerated, moving at speeds less than the speed of light. In contrast, light has no rest mass—i.e., it lacks inertia (it does not undergo acceleration or deceleration)—and always travels at about 300,000 km/s in whatever frame of reference. In the framework of special relativity, the concept of rest mass takes on a more abstract role as a relativistic scalar invariant. However, one does not need to pin down rigorous definitions to recognize that physics is replete with entities that are 'immaterial' yet nonetheless real, concrete, and not 'unphysical.'
For example, light is a massless—that is, ‘immaterial’—electromagnetic field that propagates throughout space and interacts with material particles or bodies. From the perspective of quantum electrodynamics (QED), electromagnetic waves are mediated by light particles—the massless gauge bosons, commonly known as photons—which are immaterial entities that, as we know from everyday experience, have considerable causal power over material ones.1
The same can be said about the action of a gravitational field. Gravity is a massless force field that interacts with material objects. The causal influence of this 'immaterial' field on massive bodies, such as planets and stars, is well established. In general relativity, gravity is represented as a curved 4D differentiable spacetime Riemannian manifold whose curvature is determined by mass or energy and along which everything follows the shortest geodesic path. Taylor and Wheeler summed it up concisely: “Spacetime tells mass how to move, mass tells spacetime how to curve” (Taylor and Wheeler, 1992). The status of the gravitational force relative to the three other fundamental forces (the electromagnetic and the strong and weak nuclear forces) is a matter of some debate; however, the relevant fact is that a material object can interact with another material object without direct contact by warping its surrounding vacuum.
The question of how causal influence can occur between material bodies without direct contact through empty space, solely through an immaterial field or an even more ethereal notion of spacetime curvature, is not a novel philosophical issue. When Newton was asked about the nature of gravity's 'action at a distance,' his official response in the Principia was: “Hypotheses non fingo”(“I do not frame hypotheses”). However, his private correspondence reveals that framing hypotheses was likely one of his major occupations: “That gravity should be innate, inherent, and essential to matter, so that one body may act upon another at a distance through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it. Gravity must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial, I have left open to the consideration of my readers” (Cohen, 1978).
Unfortunately for Newton, for centuries, nearly all scientists with a “competent faculty of thinking” have accepted this “great absurdity” without further questioning it.
The same phenomenon presents itself at the microscopic scale in the realm of nuclear forces. The stability of matter relies on the strong nuclear force, which binds quarks through asymptotic confinement to form protons and neutrons in atomic nuclei. This interaction is mediated by massless gluons, the gauge bosons responsible for the strong nuclear force.
Inside a proton, what we imagine as a simple structure of three quarks (two up, one down) is actually a ‘boiling’ and dynamic environment often described as a ‘quantum foam.’ This term refers to the constantly fluctuating sea of virtual particles—quark-antiquark pairs and gluons—that spontaneously appear and vanish due to quantum uncertainty. These fluctuations are governed by quantum chromodynamics (QCD), the theory of the strong force. The gluons, not only couple with the three quarks but also interact with each other, creating a dense, non-linear web of energy. Virtual quark-antiquark pairs (called the ‘quark sea’) pop in and out of existence, momentarily influencing the proton's mass, spin, and charge distribution. This complex quantum foam contributes the majority of the proton’s mass—not from the quarks themselves, but from the dynamic energy of the gluon field and virtual particles, which are themselves massless (a manifestation of energy-mass equivalence via Einstein’s famous equation E=mc^2.) In other words, your brain and body is mostly made of immaterial stuff, it’s weight results prevalently from these interactions mediated by massless particles!
The fact that massless fields can interact with material particles rarely raises concerns because modern physics takes a pragmatic approach, accepting this state of affairs as a given. Instead, it focuses on the mathematical formalization that allows for empirically verifiable predictions, largely overlooking the philosophical issues regarding how immaterial entities can causally affect material objects. No explanation regarding the principle of causal heterogeneity is deemed necessary.
Yet, for some reason, when a similar conceptual issue arises in the philosophy of mind—specifically regarding how an immaterial mind could interact with a material brain or body—it is presented as a seemingly insurmountable objection against any form of dualism.
Of course, one might object that the central issue in the debate over mental causation should not be framed as a conflict between 'material' and 'immaterial' physical entities. Instead, it involves the question of how an 'unphysical substance' can influence a 'physical substance.' Indeed, that’s what we will see next.
However, the terminological clarification was necessary. It underscores that when we deal with more foundational philosophical questions, category conflation can lead to conceptual confusion. It also highlights that a broader interaction problem has existed throughout the history of science yet has not been considered particularly worrisome.
2. What Does ‘Unphysical’ Mean?
The same question arises regarding the distinction between what is meant by something being ‘physical’ versus ‘unphysical’ or ‘metaphysical.’ When discussing whether a non-physical mind can influence a physical brain, we cannot exempt ourselves from clarifying the distinction between what ‘physical’ and ‘unphysical’ mean. It is precisely the nature of this distinction, along with the more or less unaware premises underlying it, that determines the character of our reasoning, conjectures, and conclusions. If such categories are based solely on a vague commonsense understanding of everyday experience, then speculations about the mutual interaction of physical and unphysical entities, could become misleading or devolve into mere wordplay.
For example, an unaware but not uncommon fallacy lurking behind the debates on mental causation is that of positing a priori that anything that cannot interact with the physical world is considered 'unphysical' and then asking how an unphysical mind could interact with a physical body. One begins by assuming, by definition, that anything unphysical cannot interact with the physical. Then, the mind is classified as unphysical, and then asks how this unphysical mind could interact with a physical brain. A nice trick that hides a circular reasoning such that the conclusion is implicitly assumed in the premise. While coming from the via negativa, defining physical as anything that is not mental or experiential merely reinforces the notion of the mind as non-physical and does not clarify its causal relationship either.
On the other hand, in physics there is no universally accepted definition of what constitutes 'physical,' nor is there a clear distinction between physics and metaphysics or an agreement on the foundational doctrines of physicalism. The term 'physical' varies in interpretation depending on the context and discipline, such as science, metaphysics, philosophy of mind, and philosophy of science. Nonetheless, while one could agree that physics alone cannot define the ‘physical’, one could also say that it helps us gain conceptual clarity.
My aim is not to provide a definitive answer, as, even in physics, there is no unique and rigorous characterization of the physical appropriate for mind-matter conjectures. Instead, the aim is to clarify the quantum field theoretical foundations that physics uses to describe the physical world while acknowledging its limitations. Investigating how far physics can lead us might allow us to expand (or restrict) or further define the boundaries and foundations of other metaphysical non-physicalist theoretical frameworks, an example of which I will present in the last part.
Conventional physics textbooks do not define physics in contrast to metaphysics, except perhaps through a historical note that the term 'physics'—and, thus, ‘physical’—originates from Aristotle, who coined it to mean “the study of Nature.” Nonetheless, a characterization in line with contemporary science could be to describe as ‘physical’ anything that can be empirically verified, existing in space and time, and governed by the laws of physics. Spacetime, matter (or energy, according to the mass-energy equivalence), and the laws of physics (or causality) are the ingredients of the typical understanding of classical mechanics, which is often assumed in dualist debates.
On the other hand, in physics, the term 'unphysical' does not carry a metaphysical implication; it usually refers to meaningless solutions of differential equations, such as negative or infinite energy states, complex-valued lengths or time intervals, probabilities exceeding one, the violation of symmetry principles, etc.2 An ‘unphysical solution’ is a mathematical limitation of the formalism describing impossible physical processes or nonexistent entities and does not hint at anything metaphysical.
Therefore, if we accept this definition and consider the mind to be an unphysical substance, we either refer to something that does not exist—a self-negating paradox—or imply something that transcends spacetime, matter, energy, and the laws of physics. Once again, this leads us to assume an interactive impossibility from the outset, resulting in the same circular reasoning mentioned earlier.
However, if we are looking for the most fundamental primitives of physical reality from the perspective of modern physics, this is an outdated view.
As we have seen, physics is full of immaterial things that aren’t made of matter. Matter isn’t fundamental and must be replaced by something else. The contemporary theory that describes reality at the most fundamental physical level, which is the standard model of particle physics based on QFT, describes matter, energy, forces, and all interactions with quantum fields.
So, what is a quantum field in QFT?3
Technically speaking, quantum fields are operator-valued distributions defined in spacetime that operate on a Hilbert space (or a Fock space in the context of multiparticle systems). Specifically, quantum field operators act at a given point in space and time and can be expressed in terms of creation and annihilation operators, which correspond to raising or lowering a particle’s state. These fields can be scalar fields (such as the Higgs boson) or vector fields with integer spin (bosons like photons and gluons) or half-integer spin (fermions, such as quarks and leptons). Within this theoretical framework, quantum fields are modeled as the sum of quantized harmonic oscillators (field modes), with particles representing wave-like quantized excitations of the ground state (the vacuum in its unexcited condition), with a distinct quantum field assigned to each type of particle (e.g., the photon corresponds to the electromagnetic field, while the electron is associated with the Dirac field, etc.).
However, we need not engage in technicalities; for our purposes, we can consider quantum fields to be the foundational fabric of what we refer to as 'physical' reality. In this framework, particles can be intuitively visualized as traveling-wave excitations on a spacetime manifold (sort of localized vibrating 'bumps'), with interactions occurring when these excited states couple with one another. (We will see more on the nature of interactions in the coming part).
Then, while in philosophy the concepts of ‘laws of Nature’ and ‘causality’ have also given rise to longstanding debates, in QFT the laws of physics are determined by symmetry conditions, and causality refers to the interaction of fields constrained by a relativistic microcausality condition. (No influence can travel superluminally.) All the laws of physics emerge from the requirement of symmetry principles—that is, the principle of covariance, global and local gauge invariance, and spontaneously broken symmetries. Ultimately, what we call the ‘laws of Nature’ and ‘causality’ boil down to symmetry principles involving quantum field operators constrained by algebraic rules (canonical commutation relations) imposed to preserve the wave-like aspect of the fields and particles.
Thus, according to QFT, spacetime and (wave-like) quantum fields subjected to symmetry principles and of relativistic causality are what contemporary physics considers 'physical' at the most fundamental level. In contrast, whatever transcends spatial and temporal relationships and can’t be modeled by a quantum field subjected to these symmetry and causality principles may be classified as 'unphysical.'
Is this a clear definition of what it means for something to be ‘physical’ as opposed to ‘unphysical’? Does it aid in clarifying the interaction problem between an unphysical mind and a physical brain? I will address these questions in the next post.
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References
Cohen, I., ed. (1978). “Isaac Newton’s Papers & Letters on Natural Philosophy and Related Documents.” Reprint 2014 ed. edition. Harvard University Press.
Shapiro, L. (2007). “The Correspondence between Princess Elisabeth of Bohemia and René Descartes.” University of Chicago Press; New ed.
Taylor, E.F.; Wheeler, J.A. (1992). “Spacetime Physics.” Second Edition, p. 275, New York, W. H. Freeman and Co.
One could argue that, according to the mass-energy equivalence principle, matter and energy can be converted into one another (e.g., during annihilation processes between particles and their antiparticles); their ability to interact might stem from a shared commonality at a fundamental level. However, adopting this perspective could provide more reason to embrace a panpsychist viewpoint, positing that mind and matter share a similar equivalence. This interpretation renders the interaction problem no more problematic than any interaction within physical theories.
However, it is standard practice to utilize negative energy states for the description of potentials. Moreover, history shows that, eventually, such solutions might turn out to be physical if adequately interpreted. The paradigmatic example is the negative energy states predicted by Dirac’s equation, which are now interpreted as representing antiparticles.
Whether the ultimate ontology of QFT is based on particles, fields, quanta, ‘bundles of properties,’ or whatever other kind of noumenal entities is debatable. However, despite the existence of various interpretations, the mathematical formalism of QFT is unambiguously articulated in the language of fields. Given the wider support for this perspective, I will adopt the field as the conceptual foundation of reality in QFT.
Great post, Marco, and very much needed! Debates in philosophy of mind concerning physicalism too often leave out what "physical" means, or explain it as "the laws of physics" or "nothing over and above the physical", as if those aren’t problematic and everyone is already on the same page about what those mean. But "physical" means very different things to different people.
I wonder if there's another layer of confusion that comes from a implicit belief in the ordinary “naive” view, which takes the physical to be tangible, concrete objects perceived through the senses? Put this view on top of a lingering materialism, and you get a weird idea: the fundamental quantum level is really just smaller bits of "not quite stuff" (a Matter replacement essentially) which you can somehow add together to get the world we perceive. These people have never thought of Matter as problematic, never thought of it as a theoretical entity that is never perceived through the senses. At least this is my conjecture as to why reductionism and determinism are still so popular. But who knows. I can't make sense of it.
“Is this a clear definition of what it means for something to be ‘physical’ as opposed to ‘unphysical’?”
Haha…not for me! But I wasn't even on board with Matter and materialism, and now everything's supposed to ride on quantum fields and spacetime and math! It's all too much for me to wrap my mind around. I’m still living in the naive primitive world where ‘physical’ means tangible, concrete stuff I can touch and see and smell and hear. This is the only kind of "physical" that makes sense to me.
I am told the definition of physical in philosophy of mind is "Anything that can be exhaustively quantified without reference to qualities whatsoever". So it is rendered to whether someone is good at mathematics or not and whether they're stupid enough to believe qualities are "merely epiphenomenal" and quantities are "fundamental", all independent of an observer observing or inventing those very quantities, and, somehow in the absence of a person altogether. Thus, it is Thomas Nagel's "view from nowhere".
Notice, I never said "physicalism" refers to that which sensorimotor processes or properties or faculties interact with (e.g., that which you can touch or hold in your hand). Instead, physicalism here means math equations and numbers, conveniently written under the numerical Arabic system, and such and such, that are presupposed as pre-existing before spacetime itself (e.g. Max Tegmark).
In this view, if the universe implodes or explodes from some black hole or apocalypse, somehow what will remain "eternal" are the Schrodinger equations and such and such, as it were, as disembodied Platonic forms, but also in English and Arabic numerals, and all without an observer or interpreter. That's what the mathematical physicists want you to believe is "physical".
It is extremely rare for them to admit how outrageous this is.
I submit the start of such needless debates, and their religious wars, persecutions, and other systematic betrayals over the eons, began when mathematicians contaminated the gene pool with their criteria of "quantities over qualities".
Indeed, the medievals knew more about the human condition and consciousness than we do today.