Cartesian Dualism
Descartes' Project in Brief
- Skepticism as a methodological strategy.
- Cogito, Ergo Sum
Descartes' Arguments for Dualism (Meditation 6)
Leibniz's Law
Weak Form:
For any X and any Y, IF X is identical to Y, THEN, for any property F, X has F if, and only if, Y has F.
Strong Form:
For any X and any Y, IF, for any property F, X has F if and only if Y has F, THEN X is identical to Y.
We may derive the following Corollary to the Weak Form of Leibniz's Law:
For any X and any Y, IF, for some property F, X has F but Y does not have F, THEN X is not identical to Y.
| Descartes' First Argument for Dualism | |||
| 1 | I can doubt the existence of my body, but I cannot doubt the existence of my mind. | ||
| 2 | If (1), then mind and body are distinct. | ||
| ∴ | 3 | Mind and body are distinct. | 1&2 |
| Descartes' Second Argument for Dualism | |||
| 1 | Bodies are divisible into things which are also bodies, but minds are not divisible into things which are also minds. | ||
| 2 | If (1), then mind and body are distinct. | ||
| ∴ | 3 | Mind and body are distinct. | 1&2 |
| Descartes' Third Argument for Dualism | |||
| 1 | I can clearly and distinctly conceive of existing without a body, but I cannot clearly and distinctly conceive of existing without a mind. | ||
| 2 | If (1), then mind and body are distinct. | ||
| ∴ | 3 | Mind and body are distinct. | 1&2 |
Criticisms of the Arguments
Consider the possibility of legitimate and illegitimate uses of the Corollary to the Weak Form of Leibniz's Law:
Legitimate:
Peter Parker can sling webs, but Mary Jane cannot. Hence Peter Parker and Mary Jane are not identical.
Illegitimate:
I believe that Spiderman is a hero, but I believe that Peter Parker is not a hero. Hence Spiderman and Peter Parker are not identical.
I claim that Descartes' First and Third Arguments for Dualism depend on illegitimate uses of the Corollary to the Weak Form of Leibniz's Law.
Consider again Descartes' Second Argument for Dualism:
| Descartes' Second Argument for Dualism | |||
| 1 | Bodies are divisible into things which are also bodies, but minds are not divisible into things which are also minds. | ||
| 2 | If (1), then mind and body are distinct. | ||
| ∴ | 3 | Mind and body are distinct. | 1&2 |
Note that "Body" must be taken in the sense of "physical substance" to make sense of premise (1).
Is the mind truly indivisible? What about Plato's divisions? Freud?
Descartes' Conception of the Mind
The Soul has thoughts.
Some thoughts are actions of the soul, or volitions.
Some volitions are directed to the soul itself.
Other volitions are directed to the body, and result in motion.
Other thoughts are passions of the soul, or perceptions.
Some perceptions are caused by the soul itself.
Other perceptions are caused by the body.
The Soul and the Body are distinct substances that are uniformly united.
The Soul and the Body are causally linked (somehow) at the pineal gland.
Criticisms of Cartesian Dualism
Non-explanatory
Knowing that what the mind is not (it's not physical) does not tell us what the mind is.
The Puzzle of Causal Interaction
How can two different kinds of things (one physical, one not) interact causally?
The Apparent Dependency of the Mind on the Brain
- Psychoactive Drugs.
- Brain surgery and brain damage.
- PET scans and the relationship between neural function and cognitive function.
In sum, Cartesian Dualism is deeply non-explanatory and appears to be inconsistent with (currently) known facts.
Smullyan's Objection
| Smullyan's Argument against Dualism* | |||
| 1 | If Dualism is true, then it is not possible for there to be a functional duplicate of a human being that does not possess a soul. | ||
| 2 | It is possible for there to be a functional duplicate of a human being that does not possess a soul. | ||
| ∴ | 3 | Dualism is not true. | 1&2 |
Moral of the Story: A difference that makes no difference is no difference.
*Extracted from Smullyan, R., "An Unfortunate Dualist" in Philosophy of Mind: Classical and Contemporary Readings, D. Chalmers ed., (Oxford University Press, 2002)