Substance dualism (SD)


  • [required] Section on dualism in chapter 2 of Paul M. Churchland (1984) Matter and consciousness : a contemporary introduction to the philosophy of mind MIT Press. Online preview at amazon. HKU ebook
  • [Required] Chapter 7 on dualism. In Ming, Thomas; Wong, Ching Wa; Leung, Danny; Yung, Lawrence; Lau, Yen Fong (2006) Twenty Problems in Philosophy McGraw-hill preprint version on HKU intranet
  • William Lycan (forthcoming) "Giving dualism its due"

Arguments in support of SD

The argument from doubt

  1. I cannot doubt that I exist.
  2. I can doubt that my body / brain exists.
  3. Therefore, I am distinct from my body / brain.

Compare: "Tomas is tall. Thomas is not tall. So Tomas is not the same as Thomas."

Intensional context : A predicate P(x) creates an intensional context iff it is possible that P(a) and P(b) have distinct truth-values even though a=b.

  • Not intensional: "x is tall"
  • Intensional: "Tom believes that x is tall"

The argument from indivisibility

Descartes, René. (1641). Meditations on First Philosophy. E. S. Haldane and G. R. T. Ross, trans. In The Philosophical Works of Descartes, Vol. 1. Cambridge: Cambridge University Press, 1970, p.196.

@When I consider the mind, that is to say, myself inasmuch as I am only a thinking thing, I cannot distinguish in myself any parts, but apprehend myself to be clearly one and entire.@

  1. My mind is not divisible.
  2. My body / brain is divisible.
  3. Therefore, my mind is distinct from the body / brain.


  • Is the mind really indivisible?
  • "Descartes cannot distinguish in himself any parts" => "Descartes does not have any parts"?

Arguments based upon paranormal phenomena

  1. Phenomenon X has been observed.
  2. X is best explained by SD.
  3. Therefore, SD is true.

Examples of X: testimonies of people seeing ghosts, communicating with the dead through a medium, out of body experiences

Arguments against SD

The argument from causal interaction

  1. The mind causally interacts with the physical world.
  2. SD cannot explain this causal interaction.
  3. Therefore SD is false.


  1. Parallelism - the mental and the physical do not interact.
  2. Explanatory failure does not imply falsity. Brute fact?
  3. Our best physical theories are non-deterministic, and so allows scope for non-physical causal interaction.

Empirical objections

  • SD violates physical laws such as the conservation of energy. See Main/DualismEnergy.

The argument from animal minds

  1. Whether a creature has a soul is an all-or-nothing matter.
  2. Whether a creature has a mind is not an all-or-nothing matter.
  3. Having a mind is not the same as having a soul.

Inference to the best explanation

  1. We should choose the theory that provides the best explanation.
  2. Physicalism provides a better explanation of the mind than SD.
  3. Therefore we should not accept SD.

What needs to be explained:

  • Causal interaction between the mind and the world.
  • Localization of brain functions - language, emotions, memory, personality, visual perception, facial recognition ...
    • PhineasGage
    • BrainReading

Mutations of SD

What positions can a modern-day dualist retreat to?

  1. The mind as a distributed system - mental activities take place mainly in the brain, but some still occur in the soul. (But which ones?)
  2. The soul is an emergency backup system - normally functions as a memory backup, but activates when you die. (see ConsciousnessBuddhismLevels)
  3. Idealism - The "physical" world is a mental simulation, a product of consciousness (but whose?).