The Evidence for Functionalism—On Intelligence, Consciousness, and The End of Metaphysical Excuses
We don’t need ghosts to explain minds, we need only to understand how they function and what they do.
What minds do,
is not what they are.
Philosophical debates about the nature of mind have long been dominated by abstraction, intuition pumps, and speculative thought experiments. From to behaviorist black boxes to today’s debates about AI and consciousness, the core question persists:
What constitutes a mind?
One attempt to address this was , which arose in the mid-20th century as a response to the limitations of behaviorism and the challenges of mind-body dualism. Philosophers like , , and proposed that mental states should be understood not by what they’re made of, but by what they do — by their causal roles within a system. Rather than reducing the mind to behavior or retreating into metaphysics, functionalism offered a middle path—A view where minds are real, structured, and explainable through their patterns of interaction.
In this article, rather than chasing ontological definitions, we take a pragmatic turn and focus on the key question:
What do minds actually do?
Functionalism answers this with clarity. It holds that mental states are defined by their causal roles in a system — how they relate to inputs, outputs, and internal transformations. In short, what matters is function, not form.
This article makes a rigorous, evidence-based case for functionalism. Drawing from neuroscience, evolutionary biology, and cognitive science, we argue that intelligence is functionally grounded and that consciousness, whatever else it may be, depends on that function. We will also address and dismantle the most persistent counterarguments — the so-called philosophical “ghosts” that haunt this discussion.
Functionalism grounded: the five pillars of empirical support
Convergent evolution of intelligence
Advanced intelligence evolved independently in mammals and birds , despite radically different neural architectures. Birds lack a neocortex, yet show planning, tool use, and social inference. Their pallial structures, especially the caudolaterale (NCL), serve as integrative hubs functionally analogous to the mammalian prefrontal cortex.
This pattern extends even further — cephalopods, especially octopuses, have evolved highly sophisticated nervous systems and behaviors despite diverging from the vertebrate lineage over 500 million years ago. With distributed neural networks in their arms, short and long-term memory, observational learning, and problem-solving capabilities, octopuses demonstrate intelligence that evolved through entirely separate biological pathways.
This is strong evidence that the function of intelligence does not require a specific biological form. What matters is what systems do, not what they are made of.
Neuroplasticity and functional reassignment
Stroke patients and children with can often recover cognitive abilities. In many cases, language, memory, or motor skills relocate to different regions of the brain. This extraordinary adaptability reveals that the brain does not operate via rigid, hardcoded structures, but via dynamic and flexible functional mappings.
Neuroscience confirms that brain areas are not uniquely tied to specific functions, but participate in networks that reconfigure in response to damage or demand. Functional MRI and TMS studies show how neighboring or even distant regions can take over roles with remarkable effectiveness.
This phenomenon, known as neuroplasticity , is a paradigmatic case of multiple realizability — one of functionalism’s key claims: The same mental function can be realized by different physical systems.
This is a paradigmatic case of multiple realizability — identical functions, new implementation.
Split-brain experiments
In patients, the is severed, preventing the two hemispheres of the brain from directly communicating. Intriguingly, studies reveal that each hemisphere retains distinct and specialized cognitive capacities after disconnection.
“…even though each cerebral hemisphere has its own set of capacities, with the left hemisphere specialized for language and speech and major problem-solving capacities and the right hemisphere specialized for tasks such as facial recognition and attentional monitoring, we all have the subjective experience of feeling totally integrated.” —
Despite this division, patients often report a continuous and unified sense of self. This integration is thought to be maintained by the left hemisphere’s “interpreter” — a cognitive mechanism that constructs coherent explanations for actions and perceptions, even when unaware of input from the other hemisphere.
”Indeed, even though many of these functions have an automatic quality to them and are carried out by the brain prior to our conscious awareness of them, our subjective belief and feeling is that we are in charge of our actions.” —
However, in some cases, this disconnection can lead to observable conflicts between the hemispheres, suggesting that the sense of unity is reconstructed — and at times, strained — rather than innately preserved.
This can create the appearance of two centers of consciousness (one per hemisphere), each capable of perceiving stimuli, responding to commands, and even holding differing beliefs. For example, a patient may verbally deny seeing an image presented to the left visual field (processed by the right hemisphere) while simultaneously drawing it with the left hand.
These experiments reveal that consciousness is not monolithic. It can be divided, modular, and task-specific, supporting the idea that mental states are functionally distributed across networks, not bound to any single unified core.
It reveals consciousness as a modular and distributed functional pattern.
hemispheres may originate from unilateral mutations to one hemisphere. In the example depicted here,
the left hemisphere gives up the capacity for perceptual groupings — presumably present in each
hemisphere of lower animals — as it changes to accommodate the development of language. Because
the corpus callosum connects the two hemispheres there is no overall cost to the cognitive/perceptual
system.
Cross-species cognitive parity
Crows plan multi-step solutions. Octopuses learn by observation. Bees can recognize human faces. These examples aren’t surface-level tricks; they indicate deep cognitive abilities — working memory, learning by analogy, symbolic recognition, and context-sensitive behavior.
—sketches courtesy of Marino Amodio.
What makes these findings especially compelling is their occurrence in species with dramatically different nervous systems. Cephalopods, for example, possess a decentralized brain structure, with large portions of their neural tissue distributed throughout their arms. Despite this, they engage in play, explore novelty, and demonstrate individual personalities.
This means that cognitive parity is not confined to mammals, or even to vertebrates. The computational properties of cognition appear again and again, shaped by ecological needs rather than biological lineage.
Brain-computer interfaces and prosthetics
In patients with paralysis or limb loss, intention and control are preserved and rerouted via brain-computer interfaces (BCI). With electrodes implanted in motor cortex or surface EEG systems, neural signals are translated into commands for robotic limbs, cursors, or communication devices.
What this demonstrates is that motor intention and planning are not tied to muscles or limbs. The brain’s outputs can be functionally redirected, and artificial actuators can close the loop in human-machine systems.
Studies show that over time, users can incorporate these prosthetics into their body schema, forming seamless perception-action loops. This is a powerful empirical validation of the idea that what defines an action is not its form, but the function it plays in a system of intention, feedback, and adaptation.
Function is preserved through non-biological substrates. Intention, planning, and execution do not depend on muscles or bones. They are portably functional. The pattern is clear:
Cognition is not biologically exclusive. It is functionally general. The hard data show that intelligence is functionally defined and substrate-agnostic. This is not theory-this is observed evidence.
A functional trajectory for intelligence
Max Bennett’s “A Brief History of Intelligence” outlines five evolutionary stages of cognition (see exhibit above). These stages illustrate not only the progression of cognitive function, but also how these functions have emerged independently across biological lineages, reinforcing the functionalist claim of multiple realizability:
(i) Steering: Basic motor responses that orient organisms toward or away from stimuli. Found in everything from bacteria to reptiles, this function evolved repeatedly through entirely different motor architectures.
(ii) Reinforcement: Systems that adapt behavior based on rewards and punishments. Observed in insects, mammals, and cephalopods, this function often uses distinct molecular pathways but serves the same adaptive goal.
(iii) Simulation: The internal modelling of future or hypothetical outcomes. Birds and mammals exhibit this in planning behaviors, yet their neural substrates (NCL vs. prefrontal cortex) differ substantially.
(iv) Mentalizing: The capacity to infer the beliefs or intentions of others. Corvids and primates both demonstrate this ability, suggesting convergent evolution of social cognition.
(v) Language: While uniquely developed in humans, forms of symbolic communication have evolved in dolphins, parrots, and apes. These systems differ in encoding and medium, but functionally convey structured meaning.
The convergent evolution of integrator neurons in the mammalian prefrontal cortex and avian NCL further supports the idea that recursive, representational integration is a reusable cognitive blueprint.
Evidently, we may conclude that
intelligence is not a mysterious essence. It is an evolutionary trajectory of recursively integrated functional capacities.
Consciousness and the intelligence threshold
Philosophers may dispute what consciousness is, however, it is difficult to dispute that:
there is no known case of consciousness without intelligence.
This section marks a critical shift. We move from describing intelligence as it appears in nature to asking whether consciousness (however elusive it may be), might still be bounded by the same functional contours.
Even in altered states like dreams, meditative absorption, or psychedelic experiences, we observe clear markers of intelligent structure — perception, memory, perspective, integration. Conscious states are never chaotic noise — they are functionally ordered phenomena.
Some might argue that intelligent behavior can occur unconsciously, or that consciousness includes non-functional “.” But what matters here is that every observable conscious state (including our own introspective reports), occurs in systems that exhibit integration, adaptation, and self-modelling. Whatever else consciousness may involve, it is never functionless.
This leads us to a modest but firm claim:
If intelligence is functionally grounded, and consciousness always exhibits intelligence, then consciousness must be describable in functional terms.
This doesn’t solve consciousness. But it localizes it — it confines the space in which consciousness can operate to those systems that do something — systems that integrate, represent, and recursively reweight internal states.
Consciousness may resist precise definition, but it never shows up without intelligent function.
Having grounded our claims for functionalism in evidence, and inverted the traditional relationship between intelligence and consciousness, we are now ready to confront its most persistent critics. The classic thought experiments from the likes of , and (see below) that suggest something crucial is missing from functionalism. If our argument is to stand, it must survive these philosophical ghosts.
Debunking the philosophical ghosts
Functionalism must be more than a neat theory. It must withstand challenge — not just from evidence, but from some of the most persistent and famous philosophical objections ever posed. The thought experiments discussed below are often treated as showstoppers. But when scrutinized, they are far less convincing than they appear. Here we address four of the most influential challenges and show how they collapse under the weight of their own assumptions — or their lack of empirical grounding.
In the following, I will first present for each of these categories the related hypothesis/thought experiment and then respond to that in discourse.
Inverted qualia
Hypothesis—Two individuals are functionally identical in every way (same responses, same behaviours, same linguistic reports), but experience different subjective colours (e.g., one person’s red is another’s blue).
This idea is seductive because it seems intuitively possible. But in effect it’s a philosophical mirage. There is no evidence for such a phenomenon, nor a coherent explanation of how such an internal difference could exist without any functional impact. In fact, if it truly made no difference to cognition, memory, behavior, or inference, then what exactly is being claimed to differ?
The idea presupposes a mysterious, non-functional essence to experience — a kind of ghost property immune to causal influence. But functionalism isn’t about denying experience. It’s about insisting that if something plays no role, then it belongs to metaphysics, not mind science. relies on an unprovable claim about invisible difference. It’s not a disproof — it’s a distraction.
It’s also worth noting that real-world phenomena like red-green color blindness in males often serve as mistaken analogies for inverted qualia. results in measurable differences in perception, behavior, and neural activity — it is a functional divergence, not a hidden divergence. Individuals with color blindness perform differently in color identification tasks, and their neural pathways process color information in structurally distinct ways. Far from proving the case for inverted qualia, color blindness actually reinforces functionalism by showing that changes in subjective experience are always accompanied by changes in function.
Philosophical zombies
Hypothesis—A being behaves in every way like a human (writes philosophy papers, cries at movies, reports dreams), but lacks consciousness entirely. A perfect behavioural duplicate, yet no one is home.
This is perhaps the most famous objection to functionalism. But in effect, it’s not a challenge grounded in logic or evidence — it’s an assertion of dualism, repackaged as imagination. If no function is missing, then all that’s left is a vague appeal to an undefined “something more.”
Functionalism, by contrast, is accountable. It defines mental states by what they do — how they cause and are caused by other states, behaviors, and environmental interactions. If a zombie behaves identically to a conscious human, then either consciousness is functionally present, or it’s an — a passenger with no steering wheel.
The zombie argument doesn’t reveal a flaw in functionalism. It simply assumes a reality that functionalism denies. That’s not a contradiction; it’s a metaphysical disagreement.
China brain
Hypothesis—Imagine every person in China is given a rulebook that lets them simulate the activity of a neuron in a human brain. The entire country, acting together, replicates the behaviour of a conscious mind. Is the resulting system conscious?
This thought experiment is meant to produce intuitive discomfort. It “feels wrong.” But as an argument, it fails. Why?—Because intuition is not evidence, and scale is not substance.
The shares causal structure with a conscious system. That’s all functionalism requires. If the system functions identically (integrates information, forms intentions, adapts to context), then it instantiates the same mind. Discomfort doesn’t defeat the claim. It only reveals our biases about scale, speed, and substrate.
Functionalism does not care whether the computation happens in a silicon chip, a wet brain, or a nation with a million rulebooks. If the functional roles are preserved, then so is the mind. That is the commitment — causal structure, not intuition, defines identity.
Crucially, Functionalism does not rely on this thought experiment to prove its validity — it holds up on its own empirical and theoretical grounds. But if this experiment is to be used to disprove functionalism, the burden of proof shifts — one must actually run the simulation, demonstrate that it fails, and show that function was preserved while mind was lost. Until then, the China Brain remains not a counterexample, but an intuition pump — compelling to the imagination, but unsupported in fact.
Knowledge argument (Mary the color scientist)
Hypothesis—Mary is a scientist who knows everything about colour perception but has lived in a black-and-white room. When she sees red for the first time, she learns something new. Therefore, physical or functional knowledge is incomplete.
This argument confuses having information with engaging with information. Mary may know all the third-person facts, but seeing red involves a new mode of interaction — a first-person perspective instantiated by a functioning perceptual system.
There is no magic in this. It’s the difference between knowing how a musical scale works and hearing a melody. The experience of seeing red is not an extra metaphysical fact — it is the system accessing and engaging with information in a new mode. This perceptual interaction reflects the system’s internal state change in response to new input, revealing not a gap in knowledge, but a shift in functional access. This is the emergent property of experience at work — not something beyond function, but something functionally instantiated through structure and perspective.
Sensorimotor theories, and many representationalistic models, support this idea — what changes is how the system engages with data, not what it knows in abstract. Functionalism accommodates this by recognizing that procedural and phenomenal states are modes of interaction, not disconnected domains.
Along these rationales, we can conclude:
These philosophical arguments do not disprove functionalism. They rest on intuition, metaphysics, or miscategorized epistemology.
None offer evidence that mental states can exist outside causal roles. And none demonstrate a single real-world case where functional equivalence fails to account for cognition.
Functionalism endures not because it avoids these challenges, but because it answers them without invoking ghosts.
A ghostless mind—the reasonable conclusion
Functionalism doesn’t solve consciousness. It does something better — it grounds it. Where minds adapt, integrate, simulate, and respond, we find function. And where that function is coherently organized, i.e., capable of modelling, reweighting, and acting in context, we find the architecture of the mind. Not as magic. As mechanism.
Of course, there may be more to say about awareness, about identity, about meaning. But none of it will stand if it cannot first pass the functional test.
We don’t need ghosts to explain minds. We need only to understand what minds do.
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