1 Itís possible for oneói.e., for a mindóto exist disembodied.
Therefore, Cartesian Dualism is true.
2. Itís possible for one to be a student without being a violinist.
Therefore, nothing can be both a student and a violinist.
3. A is necessarily F iff itís impossible for A to exist without being F; i.e., in every possible world in which A exists, A is F. (A is contingently F iff A is F but might not have been; i.e., although A is actually F, thereís some possible world in which A exists but isnít F.)
4. Bachelors are not necessarily bachelors (for itís possible for someone who is in fact a bachelor to exist without being a bachelor; he might, that is, get married).
5. Bodies are necessarily bodies.
6. Suppose that itís possible for you, a mind, to exist disembodied. Then it follows that youíre not a body, since, if you were a body, it would be impossible for you to exist without being a body. Since a body is anything that has physical properties, and since itís given that youíre a mindói.e., a thinking thingóit further follows that you have mental properties and no physical properties. Since this argument generalizes to every mind, Cartesian Dualism is true on the supposition that itís possible for minds to exist disembodied.
7. The causal-interaction argument:
(i) Mental and physical events causally interact.
(ii) If (i) and CD is correct, then either
a) Some physical eventsóviz., certain physical changes in the human bodyódonít have complete physical explanations, or else
b) there is superfluous causal overdetermination [superfluous causal overdetermination is what you would have if causes of a certain type never occurred except when they were accompanied by metaphysically independent causes of the same effects].
(iii) Not a), for two reasons. First, itís hard to see how human bodies can behave differently from other bodies, given that all bodies are made up of the same elements, and we evidently do suppose that all non-human physical events have physical explanations. Second, the hypothesis that the physical events in question donít have complete physical explanations appears to violate the principle of the conservation of energy, which holds that energy canít be destroyed or created; although it may be transformed from one form to another, the total amount of energy is unchangeable.
(iv) Not b), for itís simply too ad hoc to take seriously; if there were a God, sheíd be convicted of being an incompetent engineer.
(v) Therefore, CD isnít correct.