Presented by Zia H Shah MD

Please read the Epilogue at the end before you read rest of the article

Audio teaser: Does Physics Rule Out Free Will

Abstract

The tension between metaphysical materialism—frequently articulated in contemporary philosophy as physicalism—and the traditional conception of libertarian free will constitutes one of the most enduring and analytically rich paradoxes in the philosophy of mind. This comprehensive report examines the foundational architecture of physicalism, specifically focusing on the principle of the causal closure of the physical domain, to demonstrate how metaphysical materialism inevitably reduces human agency to either hard determinism or epiphenomenalism. Faced with the perceived impossibility of contra-causal freedom within a purely physical, nomologically sealed universe, a robust intellectual tradition of compatibilism has emerged. This analysis exhaustively explores the varied compatibilist frameworks proposed by David Hume, Harry Frankfurt, P.F. Strawson, and Daniel Dennett. By precisely defining their respective conceptualizations of free will, the report elucidates how these thinkers systematically dismantle the libertarian requirement for alternative possibilities and “uncaused causes.” Instead, they reconstruct freedom through the lenses of psychological spontaneity, hierarchical volitional structures, interpersonal reactive attitudes, and evolutionary competence. Finally, a thematic epilogue synthesizes these metaphysical debates, arguing that if compelling evidence for libertarian free will exists, it fundamentally shatters the causal closure principle, thereby serving as the ultimate and most decisive refutation of metaphysical materialism.

The Metaphysical Architecture of Physicalism and Causal Closure

To comprehend why metaphysical materialism inevitably forces the concession of hard determinism regarding free will, one must first rigorously delineate the foundational axioms of the physicalist worldview. Metaphysical materialism, or physicalism, posits that everything that exists is ultimately physical, and that all phenomena, including mental states, consciousness, and human volition, supervene upon or are grounded in physical properties and laws. Within this framework, the universe operates according to strict nomological principles—whether these are the deterministic laws of classical mechanics or the probabilistic laws of quantum mechanics.  

The inescapable descent into hard determinism (or its probabilistic equivalent, which offers randomness rather than control) is most rigorously demonstrated through the Principle of the Causal Closure of the Physical Domain. Jaegwon Kim, a preeminent philosopher of mind, articulates this principle as follows: “If a physical event has a cause at t, then it has a physical cause at t“. Causal closure asserts that the physical world is causally self-contained and completely comprehensive; no non-physical forces, immaterial substances, or emergent mental properties can intervene to alter the trajectory of physical events.  

When applied to the problem of mental causation and human volition, physicalism generates an intractable dilemma for any robust notion of free will. This dilemma is most potently formalized in Jaegwon Kim’s “Causal Exclusion Argument”. The exclusion argument demonstrates that if the physical domain is closed, any mental state that supposedly causes an action is either identical to a physical brain state (and therefore governed entirely by physical laws) or it is completely causally inert.  

Premise of the Causal Exclusion Argument	Philosophical Definition	Implication for Human Agency
Mental Causation	Mental events (e.g., a conscious decision) appear to cause physical effects (e.g., an arm rising).	Preserves the common-sense intuition that our thoughts drive our bodily actions.
Physical Causal Closure	Every physical event that has a cause has a sufficient physical cause.	The physical universe is a sealed system. Physics alone is sufficient to explain all physical movements.
Non-Identity (Irreducibility)	Mental properties are distinct from, and not strictly identical to, their physical realization bases.	Preserves the uniqueness of consciousness (qualia), favored by non-reductive physicalists and dualists.
Causal Exclusion Principle	No single effect has more than one sufficient cause at a given time, excluding genuine overdetermination.	If physics provides a complete causal history for an action, the mental cause is rendered redundant.
No Systematic Overdetermination	It is scientifically implausible that every voluntary action is simultaneously caused by independent mental and physical forces.	Precludes the possibility that every human action is like a firing squad where two bullets simultaneously cause a single death.

Given these interdependent premises, if a physical brain state is entirely sufficient to cause the subsequent physical action—which the causal closure of physics guarantees—the independent mental state is causally excluded. The conscious will is stripped of its causal efficacy, rendering it an epiphenomenon.  

The Inevitability of Hard Determinism and Epiphenomenalism

The causal exclusion argument illustrates why physicalism leaves no ontological space for the traditional conception of free will. Historically, this realization led to the formulation of epiphenomenalism. Thomas Henry Huxley famously illustrated this with his locomotive analogy: the relationship between the mind and the brain is like the relationship between the steam whistle on a locomotive engine and the engine itself. Just as the steam whistle is caused by the engine’s operations but has absolutely no causal influence upon the engine’s forward movement, conscious volition is a mere byproduct of neural machinery, completely lacking the power to direct behavior. As Huxley concluded, “The soul stands related to the body as the bell of a clock to the works, and consciousness answers to the sound which the bell gives out when it is struck”.  

Attempts to rescue mental causation without abandoning physicalism often fall back into hard determinism. For instance, Donald Davidson’s Anomalous Monism attempts to preserve mental causation by arguing that every mental token is identical to a physical token, thereby allowing the mental event to cause physical behavior. However, this theory succumbs to “property-epiphenomenalism.” While the event causes the action, it does so solely in virtue of its physical properties, not its mental properties. The fact that an event feels like a conscious choice adds nothing to its causal efficacy.  

If one accepts reductive physicalism—where mental states are nothing but physical states (e.g., the identity theory where pain is strictly identical to C-fiber firing)—mental causation is saved, but only at the devastating cost of acknowledging that every “choice” is fully determined by antecedent physical states, the laws of physics, and the initial conditions of the universe. Because the human agent has no control over the laws of nature or the macroscopic state of the universe prior to their existence, and because these external factors strictly entail all current neurobiological states, the agent cannot be the ultimate source of their actions. Hard determinism dictates that the belief that one is initiating a free action is merely a cognitive illusion generated by deterministic neurochemistry.  

The Libertarian Target: Unmoved Movers and Alternative Possibilities

To fully appreciate the compatibilist reconstructions of free will, one must first delineate the libertarian thesis they uniformly reject. Libertarianism regarding free will is the incompatibilist view that humans possess the capacity to make genuinely free choices that are not causally determined by antecedent events or physical laws. It relies on two primary pillars: the Principle of Alternative Possibilities (PAP) and Agent Causation.  

The Principle of Alternative Possibilities posits that a person is morally responsible for their action only if they could have done otherwise under the exact same historical, physical, and psychological circumstances. This requires a metaphysically open future where the laws of nature and the past do not dictate a single inevitable outcome. Libertarians argue that if determinism is true, the future is closed, rendering alternative possibilities impossible and destroying moral responsibility.  

Furthermore, to avoid the problem of decisions being the result of mere quantum randomness—which provides chance, not control, thereby succumbing to the “luck objection”—libertarians invoke “agent causation” or “substance causation”. This framework posits that a substance (the self, the soul, or the emergent mind) can initiate new causal chains without being determined to do so by prior physical states.  

The late Roderick Chisholm famously encapsulated this view, asserting that libertarian free will grants humans an attribute traditionally reserved for the divine: “If we are responsible, and if what I have been trying to say is true, then we have a prerogative which some would attribute only to God: each of us, when we act, is a prime mover unmoved. In doing what we do, we cause certain events to happen, and nothing—or no one—causes us to cause those events to happen”. Similarly, contemporary libertarians like Robert Kane argue for “Self-Forming Actions” (SFAs)—moments of genuine, undetermined conflict where the agent, not their physical brain chemistry, breaks the causal tie and forms their own character.  

For physicalists and compatibilists, this conceptualization is deeply problematic. It explicitly violates the causal closure of the physical domain by inserting an uncaused, immaterial agent into the mechanical operations of the brain, a move that Daniel Dennett derides as “panicky metaphysics” or “real magic”. The compatibilist project, therefore, is to discard the “prime mover unmoved” and the requirement for a metaphysically open future, while salvaging moral responsibility, autonomy, and the lived experience of choice.  

Compatibilist Theorist	Core Framework	View on Libertarian Free Will	Grounding of Moral Responsibility
David Hume	Liberty of Spontaneity	Equates it with “chance” or “indifference,” which destroys morality.	Actions must stem deterministically from internal character and motives.
Harry Frankfurt	Hierarchical Mesh Theory	Irrelevant; alternative possibilities are not required for responsibility.	Structural alignment of first-order desires and second-order volitions.
P.F. Strawson	Reactive Attitudes	Metaphysical overreach; a symptom of theoretical detachment.	The inescapable psychological framework of interpersonal human relationships.
Daniel Dennett	Evolutionary Competence	“Real magic”; an incoherent desire for uncaused causation.	Evolutionary capacity for self-control, anticipation, and avoiding harm.

David Hume and the Empiricist Reconstruction of Liberty

David Hume’s compatibilism, articulated systematically in A Treatise of Human Nature and later refined in An Enquiry Concerning Human Understanding, remains one of the most historically significant attempts to reconcile freedom with deterministic physicalism. Hume’s approach is distinctly deflationary; he argues that the entire debate over free will and determinism is a protracted semantic dispute born from a fundamental misunderstanding of the nature of causation and necessity.  

The Rejection of the Liberty of Indifference

Hume sharply distinguishes between two types of liberty: the “liberty of indifference” and the “liberty of spontaneity”. The liberty of indifference corresponds to the libertarian ideal—an absolute negation of causal necessity. Hume rejects this outright, arguing that to remove necessity from an action is to remove causation entirely, which “is the very same thing with chance”. In the Treatise, he suggests that “liberty and chance are synonimous”.  

For Hume, libertarian free will would paradoxically destroy moral responsibility rather than preserve it. If human actions were truly uncaused (indifferent), they would be random, erratic, and utterly disconnected from the agent’s character, desires, and beliefs. If an action does not flow necessarily from an agent’s character, the agent cannot be praised or blamed for it. As Hume observes: “…where they [actions] proceed not from some cause in the character and disposition of the person, who performed them… it is impossible he can, upon its account, become the object of punishment and vengeance”. Moral evaluation, therefore, absolutely requires a deterministic, necessary link between the internal psychology of the agent and their external physical actions.  

The Redefinition of Causal Necessity

Hume’s reconciliation of freedom and determinism relies entirely on his radical empiricist critique of causality. He observes that humans perceive no hidden “power” or “necessary connection” in the physical world; rather, causality is nothing more than the “constant conjunction of like objects and the inference of the mind from one object to another”.  

When observers look at human behavior, they see the exact same constant conjunction between human motives and human actions that they observe between colliding physical objects. “Moral evidence is,” Hume states, “nothing but a conclusion concerning the actions of men, deriv’d from the consideration of their motives, temper and situation”. Therefore, the “necessity” that governs the physical world also governs the human mind. Hume famously states his compatibilist position by claiming he changes nothing about human behavior, but only corrects our flawed understanding of physics: “But I ascribe to matter, that intelligible quality, call it necessity or not, which the most rigorous orthodoxy does or must allow to belong to the will. I change, therefore, nothing in the receiv’d systems, with regard to the will, but only with regard to material objects”.  

The Liberty of Spontaneity

In place of the incoherent liberty of indifference, Hume champions the “liberty of spontaneity,” which he calls the “most common sense of the word” and the “only… species of liberty, which it concerns us to preserve”. This liberty is simply the capacity to act according to the determinations of one’s own will without external coercion, physical restraint, or violent compulsion. A prisoner in chains lacks the liberty of spontaneity; a person walking freely down the street possesses it. For Hume, as long as the causal chain flows unobstructed through the internal desires and character of the agent, the agent is acting freely. This holds entirely true regardless of the fact that those internal desires are themselves the deterministic products of prior causes, nature, and socialization. In this way, Hume seamlessly integrates human agency into the causal closure of the physical world.  

Harry Frankfurt and the Hierarchical Mesh Theory

While Hume focused on the absence of external physical constraints, Harry Frankfurt shifted the compatibilist focus inward, providing a microscopic analysis of the internal psychological structure of the human will. In a series of highly influential papers, beginning with “Alternate Possibilities and Moral Responsibility” (1969) and “Freedom of the Will and the Concept of a Person” (1971), Frankfurt systematically dismantled the libertarian reliance on the Principle of Alternative Possibilities (PAP) and proposed a structural theory of human autonomy that is entirely blind to determinism.  

The Defeat of the Principle of Alternative Possibilities

Frankfurt profoundly challenges the libertarian assertion that moral responsibility requires the metaphysical ability to have done otherwise. He introduces a class of intricate thought experiments, now universally known in the literature as “Frankfurt cases,” designed to permanently decouple moral responsibility from alternative possibilities.  

Consider the canonical case of Jones and Black. Black is a nefarious neurosurgeon who wants Jones to perform a specific action, such as voting for a particular political candidate. Black implants a sophisticated device in Jones’s brain that allows Black to monitor Jones’s neurological activity and intervene only if absolutely necessary. Black prefers to avoid showing his hand. He waits until Jones is about to make a decision. If Jones decides, entirely on his own, to vote for the candidate Black desires, Black does nothing. However, if Jones shows neurological signs of deciding to vote for the opponent, Black will activate the device, overriding Jones’s neural pathways and forcing him to vote for Black’s preferred candidate.  

In the actual sequence of events, Jones deliberates and, for his own completely internal reasons, decides to vote for the candidate Black wanted him to vote for. Black’s device remains dormant. In this scenario, Jones is intuitively morally responsible for his action. He acted exactly as he wanted to act, for his own reasons, without any actual interference. Yet, due to the presence of the fail-safe device, Jones could not have done otherwise. Frankfurt uses this elegant demonstration to argue that: “It would be quite unreasonable to excuse Jones 4 for his action… on the basis of the fact that he could not have done otherwise. This fact played no role at all in leading him to act as he did”. By successfully divorcing moral responsibility from the capacity to do otherwise, Frankfurt neutralized one of the primary arguments for libertarian free will.  

First-Order Desires, Second-Order Volitions, and the Concept of a Person

Having discarded PAP, Frankfurt required a new mechanism to explain what constitutes a genuinely free action. In “Freedom of the Will and the Concept of a Person,” he introduces a hierarchical theory of desires.  

Humans and animals alike possess “first-order desires”—basic drives to act, such as the desire to eat, sleep, or consume a narcotic. However, humans are uniquely capable of reflective self-evaluation, allowing them to form “second-order desires”—desires about their own desires. When a second-order desire explicitly wishes for a specific first-order desire to become the “effective will” (the desire that translates into actual action), Frankfurt categorizes this as a “second-order volition”.  

Frankfurt makes a vital distinction between “persons” and “wantons”. A wanton is a creature (like a non-human animal or a very young child) that acts purely on first-order desires without any reflective evaluation. A wanton addict simply experiences a physiological craving for a drug and takes it; they have no higher-level opinion regarding their addiction. A person, however, possesses second-order volitions and cares about the structure of their own will.  

Frankfurt illustrates this with the tragic figure of the “unwilling addict.” The unwilling addict has a strong first-order desire for the drug, but also a conflicting first-order desire to abstain. Crucially, the unwilling addict possesses a second-order volition that the desire to abstain be the one that triumphs. If the physiological craving overpowers the addict, they take the drug despite their second-order volition. In this case, the addict acts, but does not act freely, because their effective first-order desire directly contradicts their second-order volition. Their will is alienated from their higher-level self.  

For Frankfurt, free will is achieved when there is a harmonious “mesh” between the hierarchical levels of desire. When an agent acts on a first-order desire, and they possess a second-order volition endorsing that very desire, they exercise free will. It is the capacity to want what one wants to want. This structural alignment is completely compatible with causal determinism, as the entire hierarchy of desires, including the capacity for self-reflection, can be the complex product of deterministic neurobiological processes and environmental conditioning.  

P.F. Strawson and the Inescapability of Reactive Attitudes

While Frankfurt sought to meticulously redefine the psychological mechanics of free will, the British philosopher P.F. Strawson radically reoriented the debate by shifting it away from metaphysics entirely. In his seminal 1962 paper “Freedom and Resentment,” Strawson argued that the traditional debate between the “optimist” (compatibilist/consequentialist) and the “pessimist” (incompatibilist/libertarian) is fundamentally misguided because both parties mistakenly believe that our practices of holding each other responsible require a theoretical, metaphysical justification.  

The Participant Attitude vs. The Objective Attitude

Strawson’s argument hinges on a careful phenomenological description of human interpersonal life. He observes that human beings are deeply immersed in a web of ordinary interpersonal relationships, which inherently and inescapably generate “reactive attitudes” such as resentment, gratitude, indignation, guilt, and forgiveness. When someone intentionally offends us, our natural emotional reaction is resentment; when someone benefits us, it is gratitude. These attitudes constitute what Strawson terms the “participant attitude”.  

Occasionally, due to specific circumstances, we suspend these reactive attitudes and adopt an “objective attitude” toward another person. To take the objective attitude is to view the individual not as a moral agent to be reasoned with, but as an object of social policy, “as a subject for what, in a wide range of sense, might be called treatment … to be managed or handled or cured or trained”. We adopt the objective attitude toward young children, individuals with severe mental illness, or people operating under extreme psychological stress. In these cases, we do not feel moral resentment toward their harmful actions because we recognize an abnormality or impairment in their agency.  

The Practical Irrelevance of Determinism

The incompatibilist pessimist argues that if determinism is true, everyone is fundamentally akin to the psychologically constrained individual; therefore, we must universally adopt the objective attitude, abandon all reactive attitudes, and recognize that no one truly deserves praise or blame.  

Strawson counters this by arguing that it is psychologically, emotionally, and practically impossible for human beings to universally abandon the participant attitude. The reactive attitudes are hardwired into our social nature and are constitutive of our humanity. As Strawson states: “The human commitment to participation in ordinary inter-personal relationships is, I think, too thoroughgoing and deeply rooted for us to take seriously the thought that a general theoretical conviction might so change our world…”. No abstract metaphysical proposition regarding the causal closure of physics or the truth of determinism could genuinely compel us to view our friends, lovers, and enemies purely as deterministic mechanisms.  

Moreover, Strawson argues that the pessimist fundamentally misunderstands the specific local conditions under which we suspend reactive attitudes. We excuse an agent (e.g., recognizing they were pushed, or acting in ignorance) because they did not manifest ill will toward us, not because their actions were deterministically caused. We exempt an agent (e.g., recognizing severe schizophrenia) because they lack the capacity for normal moral relationships, not because they are subject to physical laws.  

Determinism does not imply that no one has ill will, nor does it imply that everyone is mentally incapacitated. Therefore, the truth of determinism is logically and practically irrelevant to the deployment of the reactive attitudes. For Strawson, moral responsibility is not a metaphysical property of an action, but a socially constructed reality rooted entirely in our emotional responses. As he succinctly summarizes: “the existence of the general framework of [reactive] attitudes itself is something we are given with the fact of human society. As a whole, it neither calls for, nor permits, an ‘external’ rational justification”. The libertarian quest for a “prime mover unmoved” is dismissed by Strawson and his interpreters as “metaphysical megalomania”.  

Daniel Dennett, Patricia Churchland, and the Evolutionary Neurophilosophy of Competence

Modern physicalists, armed with evolutionary biology, cognitive science, and neuroscience, have continued the compatibilist tradition by aggressively naturalizing free will. They treat it not as a magical exemption from physical laws, but as a complex biological adaptation. The most prominent contemporary architects of this view are philosopher Daniel Dennett and neurophilosopher Patricia Churchland.

Dennett and the Evasion of “Real Magic”

In his extensive writings on the subject, particularly Freedom Evolves (2003), Daniel Dennett mounts an aggressive defense of compatibilism by explicitly framing libertarian free will as an incoherent desire for the supernatural. Dennett argues that when people desire free will, they often desire something that cannot possibly exist in a physical universe. He uses the metaphor of Dumbo the elephant, who falsely believes a magic feather allows him to fly. Libertarians cling to the “magic feather” of uncaused causation. Dennett famously states: “Real magic, in other words, refers to the magic that is not real, while the magic that is real, that can actually be done, is not magic”.  

For Dennett, free will is not about breaking the causal chain; it is fundamentally about control and competence. Dennett draws a strict distinction between causation and control. He explains: “The past does not control you. It causes you, but it does not control you… If you fire a gun, once the bullet leaves the muzzle, it is no longer in your control. Once your parents have launched you, you are no longer in their control”. Humans are self-controllers, continuously generating internal feedback loops that adjust behavior based on environmental data.  

Instead of metaphysical freedom, Dennett defines free will as a set of highly evolved cognitive competencies: the ability to anticipate the future, model counterfactual scenarios, avoid harm, and respond to reasons. He describes humans as “informavores,” epistemically hungry seekers of information designed by evolution to navigate a subjectively open world.  

In this framework, determinism is actually necessary for free will. As Dennett quips: “Determinism is the friend, not the foe, of those who dislike inevitability”. If the world were indeterministic, our attempts to control our environment and our behavior would be constantly thwarted by random quantum fluctuations. Determinism provides the reliable physical substrate upon which complex, rational agents can build predictive models to navigate the world effectively. Thus, free will is an “explicable achievement” aided by society and learning, fully compatible with the physical mechanisms of the brain.  

Churchland and the Neurophilosophy of Control

Patricia Churchland echoes this sentiment from a strictly neurobiological perspective in her pioneering work on neurophilosophy. For Churchland, the brain is undeniably a causal machine, entirely constrained by the laws of chemistry and physics. The libertarian illusion of an uncaused soul or an unmoved mover must be discarded in favor of a biological understanding of agency.  

Like Dennett, Churchland anchors her compatibilism in the concept of neurobiological control. Neuroscience demonstrates that different neural circuits govern automated responses (like a spinal reflex) versus deliberate, goal-directed behavior. When the prefrontal cortex successfully inhibits impulses and allows the organism to navigate complex social and environmental variables based on past learning and future modeling, the organism is exercising control. Free will, in the neurophilosophical sense, is simply the healthy functioning of these executive control networks in the brain. It is a spectrum of biological competence, entirely contained within the causal closure of the physical domain. Any attempt to ground retributive justice or absolute moral desert in a non-physical substance is, for Churchland, rendered obsolete by the realities of neuroscience.  

Thematic Epilogue: Libertarian Free Will as the Ultimate Refutation of Materialism

The exhaustive theoretical efforts of Hume, Frankfurt, Strawson, Dennett, and Churchland vividly illustrate a profound metaphysical truth: to maintain the integrity of metaphysical materialism and the causal closure of the physical domain, the traditional, intuitive conception of libertarian free will must be fundamentally altered, downgraded, or explained away. Compatibilism, for all its philosophical elegance and practical utility in preserving moral responsibility, is essentially an exercise in lowering the ontological bar. It redefines freedom as spontaneity, structural mesh, social convention, or neurobiological competence, precisely because physicalism cannot logically tolerate an uncaused cause.

However, if one assumes the opposing stance—that humans genuinely possess libertarian free will, or that there is compelling introspective, rational, or phenomenological evidence for its existence—then the theoretical paradigm violently shifts. The confirmed reality of libertarian free will would stand as the absolute strongest refutation of metaphysical materialism ever conceived.

Recall the central pillar of physicalism: Jaegwon Kim’s formulation of the causal closure of the physical domain, which demands that every physical event has a sufficient physical cause. If an agent possesses the capacity Chisholm describes—the ability to act as a “prime mover unmoved”—then a physical event (e.g., the firing of motor neurons leading to the raising of an arm) is initiated not by a preceding chain of purely physical determinants, but by an irreducible, immaterial act of volition.  

If mental causation involves genuine “downward causation,” where a non-physical mind or an irreducibly emergent self alters the trajectory of physical particles in the brain, the principle of causal closure is instantaneously falsified. As noted by philosophers of emergence like William Hasker, robust libertarian free will requires an emergent dualism or ontological pluralism that physicalism explicitly outlaws. The existence of strong emergence, where the mind possesses causal powers not found in its constituent physical parts, creates a fatal rupture in the physicalist worldview.  

The existence of a self that stands outside the deterministic web of physics to inject novel causal chains into reality means that the universe is not causally sealed. The physical domain would be porous, subject to the interventions of conscious agency. Therefore, the compatibilist project inadvertently maps the exact perimeter of the materialist fortress: if the wall of causal closure is breached by a single, genuine act of libertarian free will, the entire edifice of metaphysical materialism collapses, giving way to a universe far richer, more mysterious, and ontologically diverse than the mechanistic worldview allows.