How can the private world of subjective experience, your feelings, thoughts, and hopes, be defined in terms of the cold, hard data of objective brain science? Philosopher of mind Joseph Levine named this theoretical chasm with practical consequences the “Explanatory Gap.” It’s a core challenge for neuroscience, signifying a fundamental, possibly unsolvable puzzle at the heart of what it means to be human and have consciousness.
The Philosophical Underpinnings
At the heart of the Explanatory Gap lies the following assumption that shapes modern science: we can understand the nature of the world objectively and through physical laws. This leads us to believe that everything in the world can be understood and explained in physical terms.
This assumption is well supported in neuroscience, both in terms of behavior and its underlying physiological processes. It encounters difficulty when we consider our first-person perspective or subjective experience. Our mental states have properties that physical states do not. They appear to be subjective, personal, intentional (i.e., about something), and seemingly non-spatial; they do not appear to occupy physical space.
We take our subjective experience to be real, yet we do not know whether it is even possible to understand or examine it objectively, at least not within the traditional conception of scientific objectivity, which aims to remove personal biases and ensure replicability. And if we want to retain these kinds of properties for the mind, then this insinuates a particular dualistic nature of mental states, which goes against physicalism, which forms the backbone of neuroscience.
The Explanatory Gap in Scientific Practice
This philosophical puzzle can manifest in various forms, including the mind-body problem and the interface problem between psychology and neuroscience.
While psychology operates within the intentional, first-person mental realm, neuroscience resides in the third-person, physical domain of neurons and synapses. The Explanatory Gap suggests that even a complete physical map of the brain fails to explain why or how those physical processes give rise to the subjective “feel” of consciousness. Philosophers of mind call this qualia.
Consequently, the integration of these two fields remains fundamentally stalled by a conceptual mismatch. Because we lack a bridge law connecting neural firing to subjective experience, we often rely on neural correlates of consciousness. In this paradigm, we can observe that a specific brain state coincides with a specific thought, but we cannot explain the causal mechanism between them. If this gap is indeed an epistemic limit of human reason, we may be destined to observe the dance between the physical and the mental without ever truly understanding the music.
Explanatory Levels and Reductionism
One way to conceptualize the challenge posed by the Explanatory Gap is through the lens of explanatory levels. In science, different disciplines presumably operate at distinct explanatory levels, such as biology, chemistry, and physics. While phenomena at higher levels can be reduced to lower-level ones, doing so may not always be practical or informative.
For instance, attempting to reduce biological entities like cells to the language of subatomic particles would be conceptually cumbersome. While such a reduction would technically account for every physical property of the cell, it would yield a far poorer explanation of its life-sustaining functions. We would gain mathematical precision at the cost of the biological narrative, trading an understanding of how a cell lives for a mere list of how its atoms behave.
In brain science, the reduction from the mental to the physical appears to discard the mental properties we assign to mental states. Mental concepts can be correlated with certain neural states, but this reduction offers little explanatory power. It lacks any sort of explanation for the phenomenal experience of it or its intentionality. Yet, as part of the physical, embodied systems, physical properties must either be instantiated somewhere or be byproducts with no causal effect.
We are left with a striking paradox: while the mental appears irreducible to physical parts, the doctrine of physicalism insists it cannot be a fundamental property of the universe.
Implications for Clinical Practice
The decoupling between mental and physical levels of explanation is most visible in the lack of theoretical cross-pollination between psychology and neuroscience, as we hinted above. Cognitive neuroscience can be seen as a bridge between the two, yet it struggles with dualistic concepts that are incompatible with its physicalist assumptions.
Even psychiatry, a field theoretically grounded in neurobiology, struggles with this decoupling. Because our neuroscientific understanding of mental health often lags behind the immediate needs of pharmacology, drug development frequently relies on clinical pragmatism rather than mechanistic theory. In practice, “what works” takes precedence over “how it works.” This creates a paradigm in which treatments are based on correlations between a chemical agent and a patient’s subjective report, effectively bypassing the underlying physical mechanism and leaving us with data that lacks true explanatory power.
It is worth noting that the Explanatory Gap manifests as a conflict between researchers’ and practitioners’ theoretical commitments. While neuroscientists typically lean toward physicalism, practicing psychologists and psychiatrists often embrace mind-body dualism that aligns with the common-sense notion that the mind is distinct from the brain.
Attempts to Bridge the Gap
This professional friction is not merely a methodological curiosity but a direct consequence of the underlying metaphysical deadlock that defines the Explanatory Gap. The apparent irreducibility of the mental creates a central philosophical tension: if consciousness cannot be reduced to physical parts, yet the doctrine of physicalism insists it cannot be a fundamental feature of reality, we are left with a striking paradox. To resolve this, many philosophers turn to monist metaphysics, which seeks to unify reality under a single substance rather than maintaining a dualistic split between mind and matter.
Idealism offers a radical solution by collapsing the physical into the mental; it posits that reality is ultimately grounded in consciousness, rendering the physical world a mere manifestation of mental activity. Panpsychism (specifically Russellian Monism) provides a more nuanced path, suggesting that the physical and the mental are simply two aspects of the same fundamental stuff.
While these approaches address certain aspects of the mind-body problem, even if they could solve the hard problem of consciousness, they still offer no solution to the core problem: reconciling the subjective-objective chasm in a way that satisfies empirical science. For this reason, other theoretical or metaphysical frameworks seek to explain the mind by focusing on its structure rather than its underlying substance.
Functionalism sidesteps traditional metaphysical debates about the underlying substance or property by defining mental states in terms of their functional roles, thereby granting the mind genuine causal influence. This framework provides a clear mechanistic bridge to physicalism, as functions are anchored in neurobiological processes.
While this approach effectively accounts for intentionality by linking mental states to their sensory triggers, it remains largely silent on the subjective character of phenomenal experience. Because a system might theoretically maintain its functional integrity even if its internal feelings were altered or entirely absent, the qualia remain outside the reach of functionalist explanation.
Emergentism proposes a middle path, suggesting that while the mind is rooted in the physical brain, it is not identical to it. Instead, consciousness is an emergent property: a high-level phenomenon that only appears when physical components reach a specific threshold of complexity.
In sum, Idealism and Panpsychism attempt to resolve the gap by baking the mind into the foundation of reality, while Functionalism and Emergentism attempt to bridge it by explaining how physical systems can produce non-physical properties.
The Persistence of the Explanatory Gap: Naturalism and Its Limits
The Explanatory Gap is often framed as the philosophical problem indicative of the lack of a causal link between the mental and the physical. However, this is merely a symptom of a deeper crisis. If we cannot explain consciousness in terms of physical laws, it risks being relegated to an epiphenomenal status, that is, a powerless byproduct of the brain. However, the core of the dilemma is not exclusive to physicalism. It is a fundamental challenge to naturalism itself: our deep-seated belief that all phenomena must be objectively explainable. Subjectivity, by its very nature, leaves no objective mark that can be measured beyond spurious correlations.
Even under the frameworks of dualism, idealism, or panpsychism, the explanatory gap remains formidable. If we assume everything is experience (idealism) or that experience is a fundamental property (panpsychism), we still face a chasm between the private, subjective “I” and the public, objective “They.” In a dualistic model, this gap simply migrates to the mysterious interface where two radically different substances interact.
Consequently, as Joseph Levine initially suspected, the explanatory gap may be insurmountable. While Levine holds out hope for the evolution of new conceptual tools to bridge this divide, others, such as Colin McGinn, offer a more sobering conclusion. McGinn’s philosophical position, called Mysterianism, holds that the mind-body problem is unsolvable because of our inherent cognitive closure. Just as a rat lacks the neural architecture to comprehend calculus, the human mind may be biologically limited in its ability to reconcile its own subjective nature with objective reality. We may indeed be living in a rat maze, then.
