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Descartes’s Substance Dualism vs Functionalism

Descartes’s Substance Dualism and Functionalism, compared field by field from their entries in the Atlas.

At a glance

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Dualism Descartes’s Substance Dualism René Descartes
Materialism Functionalism Ned Block, J.J.C. Smart, Janet Levin

The claim

In brief
Descartes’s Substance DualismRené Descartes holds that consciousness, which he calls thought (cogitatio), is the essence of an immaterial thinking substance (res cogitans) wholly distinct from extended, mechanical body (res extensa). Every conscious state, from reasoning and willing to pain, color, and emotion, is a mode of the thinking thing and is immediately present to it. Sensations and passions, though, arise only in the union of mind and body, where brain processes stand in regular relations to experiences they do not resemble.
FunctionalismFunctionalism posits that mental states are defined by their causal relations to one another, to sensory inputs, and to behavioral outputs, independent of the physical medium (substrate) in which they are realized. The essence of consciousness is determined by how mental states function or operate, rather than by what they are materially composed of.
What consciousness is
Descartes’s Substance DualismConsciousness is thought, defined as everything that happens within us insofar as we are immediately conscious of it, and it is the principal attribute of the mind.
FunctionalismMental states are identified as functional states, constituted by their causal relations, roles, results, and outputs, implying they are realizable in any substrate that supports those functions.

Big questions

Can AI be conscious?Differ
Descartes’s Substance DualismNoSince thought belongs only to immaterial substance and matter is pure extension, a machine, however complex, would be mechanism without a mind.
FunctionalismYesFunctionalism holds that consciousness is based on function, not biology. If the correct functions can be realized computationally in non-biological substrates, AI consciousness is possible in principle and certain if Materialism is true.
Does consciousness survive death?Differ
Descartes’s Substance DualismYesDescartes presented the real distinction of soul and body as what makes the soul’s survival intelligible, and the Meditations announce that aim in their subtitle.
FunctionalismConditionalAs a physicalist theory, consciousness is tied to the substrate. Survival beyond death of the biological body is only possible if the functional mental state can be successfully transferred (uploaded) to a new, continuing substrate (virtual immortality).
Could a mind be uploaded?Differ
Descartes’s Substance DualismNoCopying the body’s mechanism would not copy the immaterial thinking substance.
FunctionalismLikelyFunctionalism serves as the theoretical basis for virtual immortality. If the functional role of the mental self can be perfectly duplicated and uploaded to a new medium, the first-person mental self is preserved.
Does it give life meaning?Differ
Descartes’s Substance DualismCosmic/UltimateDescartes’s system rests on a non-deceiving God who guarantees knowledge and created even the eternal truths.
FunctionalismHumanistic onlyFunctionalism describes the mechanism by which mind/consciousness operates within a physical world, providing capacities for agency and reflection, but offers no rationale for ultimate cosmic meaning or teleology, aligning with the general stance of Materialism.

Nature of mind

Ontology
Descartes’s Substance DualismSubstance dualism (Cartesian)
FunctionalismPhysicalism (broadly, agnostic to substance). The theory is classified under Materialism, but it is metaphysically agnostic regarding nonphysical substances, focusing primarily on the functional description of mental states.
Mind and body
Descartes’s Substance DualismMind and body are really distinct finite substances with exclusive essences, thought and extension, and could by divine power exist apart. They are joined in the human being, where bodily motions give rise to sensations and the mind moves the body, though Descartes never explained that interaction satisfactorily.
FunctionalismMental states are functional states identical to the functional roles that are realized or instantiated in a physical substrate, leading to 'functional state identity theses'.
Fundamental or emergent
Descartes’s Substance DualismPrimitive
FunctionalismEmergent. Consciousness arises from the functional organization realized by a sufficiently complex substrate (like neural tissue or computer chips).
Qualia
Descartes’s Substance DualismSensory qualities are not properties of matter, which has only size, shape, motion, and arrangement. Experienced redness or pain is a mental mode arising in the embodied union and cannot be captured by extension.
FunctionalismThe theory traditionally struggles to account for phenomenal qualia, often addressing them by asserting that the qualitative feel is entirely exhausted by the functional role, or by coupling with representational theories.

How it works

Mechanism
Descartes’s Substance DualismMechanism on the bodily side and non-reduction on the mental side. Light mechanically affects eyes, nerves, and brain, and the mind then experiences color, distance, and shape in regular correspondence with those motions.
FunctionalismThe principle of multiple realizability, stating that mental state realization is independent of the physical substrate, contingent only upon the required function being instantiated.
Where it is
Descartes’s Substance DualismThe mind is unextended. Sensations and passions are felt as belonging to one’s own body, and Descartes located mind-brain communication in the pineal gland.
FunctionalismDistributed wherever the functional organization is successfully realized, regardless of whether the substrate is neural tissue or hardwired computing components.
What it does
Descartes’s Substance DualismPassions are caused and strengthened by movements of the animal spirits and dispose the soul toward volition and action. The will can in turn move the body.
FunctionalismFunctions, roles, and outputs are dispositive. Mental states are causally effective because they are defined by their causal relations within the system (functional role).

Evidence and criticism

Testability
Descartes’s Substance DualismConceptually testable
FunctionalismConceptually and empirically testable. Thought experiments (like substrate replacement) probe its conceptual limits, and computational realization (AI) tests its empirical application.
Tests so far
Descartes’s Substance DualismNot stated
FunctionalismThe silicon chip replacement thought experiment is used conceptually to test whether phenomenal consciousness is preserved when the functional role is maintained across a change in substrate.
Criticisms
Descartes’s Substance DualismPrincess Elisabeth of Bohemia asked how an immaterial soul could move a material body, and the interaction problem remains unresolved. Arnauld and later critics charged circularity, since the proofs of God that guarantee clear and distinct perception themselves rely on clear and distinct perception. The standard reading denies consciousness to animals, which most now find implausible.
FunctionalismIt is criticized heavily for failing to account for phenomenal consciousness (qualia) by describing only the functional role, leading to the 'hard problem' challenge that functional roles may not exhaust the nature of conscious experience.

Sources

Key sources
Descartes’s Substance Dualism
  • Meditations on First Philosophy, René Descartes (trans. J. Cottingham, R. Stoothoff, and D. Murdoch) (1984)
  • Principles of Philosophy, René Descartes (trans. J. Cottingham, R. Stoothoff, and D. Murdoch) (1985)
  • The Passions of the Soul, René Descartes (trans. J. Cottingham, R. Stoothoff, and D. Murdoch) (1985)
Functionalism
  • What is functionalism? (N. Block) (1980)
  • What Is Functionalism? Consciousness, Function, and Representation (N. Block) (2007)
  • Functionalism (J. Levin) (2023)

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