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Descartes’s Substance Dualism vs Sam Harris

Descartes’s Substance Dualism and S. Harris’s Mystery of Consciousness, compared field by field from their entries in the Atlas.

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Dualism Descartes’s Substance Dualism René Descartes
Challenge S. Harris’s Mystery of Consciousness Sam Harris

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.
Sam HarrisThis perspective, offered as a challenge rather than a prescriptive theory, frames consciousness as an unbridgeable mystery that cannot be solved by purely physical means. Harris argues that physical descriptions are 'mute' regarding subjective experience; the only thing attesting to sentience is consciousness itself, creating a fundamental explanatory gap. He remains pessimistic that science can dispel this mystery.
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.
Sam HarrisConsciousness is identified as subjectivity or sentience, which is fundamentally mysterious and unexplainable by purely physical processes.

Big questions

Can AI be conscious?Both agree
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.
Sam HarrisNoIf consciousness is irreducible subjectivity missing from physical descriptions, then advanced computation (AI, which is purely physical complexity) will fail to generate genuine sentience.
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.
Sam HarrisNoAs a naturalist, Harris implicitly aligns with the view that consciousness is tied to the physical organism, meaning its termination results in permanent non-existence, despite the mystery of its nature while alive.
Could a mind be uploaded?Both agree
Descartes’s Substance DualismNoCopying the body’s mechanism would not copy the immaterial thinking substance.
Sam HarrisNoIf consciousness cannot be explained by purely physical processes, then duplication or uploading of the physical substrate will fail to capture or recreate the essential subjective self.
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.
Sam HarrisHumanistic onlyThe mystery does not imply cosmic purpose, but Harris advocates finding personal truths through introspection, aligning with self-discovered (naturalized) spirituality and meaning.

Nature of mind

Ontology
Descartes’s Substance DualismSubstance dualism (Cartesian)
Sam HarrisSkeptical/Mysterianism (Justified: The view asserts the philosophical intractability and unbridgeable nature of the mind-body problem, aligning with core tenets of mysterianism, though without adopting a specific alternative ontology.)
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.
Sam HarrisUnbridgeable Gap / Non-reductive Correlation (Justified: Harris asserts that while physical processes correlate with consciousness, no analysis of these processes can logically yield a picture of consciousness, leaving an unbridgeable gap.)
Fundamental or emergent
Descartes’s Substance DualismPrimitive
Sam HarrisMystery (Justified: Harris declines to fully endorse emergence, stating that while consciousness may be the lawful product of unconscious processing, he does not find that sentence meaningful in explaining sentience.)
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.
Sam HarrisQualia (what it is 'like something' to be) are the phenomenon that physical events are 'mute' about, thus creating the unbridgeable mystery.

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.
Sam HarrisNone (Justified: The core premise is the failure of all current and foreseeable physical/mechanistic principles to explain subjectivity.)
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.
Sam HarrisUnknown/Mysterious (Justified: While consciousness correlates with the brain, the theory explicitly rejects confining the explanation of consciousness to physical location or distribution because the physical world yields no evidence of consciousness.)
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.
Sam HarrisUnknown/Irreducible (Justified: Causal power is not explained by unconscious complexity. The presence of sentience is an ultimate mystery rather than a functional adaptation that can be described reductively.)

Evidence and criticism

Testability
Descartes’s Substance DualismConceptually testable
Sam HarrisConceptually Untestable (Justified: If the gap is unbridgeable, as Harris claims, the mystery cannot be dispelled by empirical science as currently practiced.)
Tests so far
Descartes’s Substance DualismNot stated
Sam HarrisNone (Justified: The framework is a diagnosis of science's failure to address the hard problem and does not propose empirical tests for the mystery itself.)
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.
Sam HarrisIt functions primarily as a critique against reductionism and scientific optimism regarding consciousness, classifying the issue as an ultimate mystery, which reductionists reject as pessimism (mysterianism).

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)
Sam Harris
  • The mystery of consciousness (Part I & II), Sam Harris (2011)
  • Explorations in Consciousness: Death Psychedelics and Mystical Experience (2023)

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