In her correspondence with Descartes, Elisabeth presented her doubt for Cartesian dualism. In particular, she poses a model of physical interaction, which would invalidate Descartes’s idea that the mind and body could interact if true:
- For a body to move, one of three things is needed:
- impulse (e.g. a spring projecting an object)
- collide (e.g. a ball hits another ball)
- displace (e.g. gears turning)
- Movement by Impulse and collision requires physical contact (and solidity)
- Displacement requires extension
- The mind/soul cannot conduct physical contact and has no extension
- Therefore, soul cannot move body.
Note
Elisabeth’s model of physical interaction is not complete. For example, Descartes raised a point about how this model doesn’t capture gravity (i.e. the model is “epistemologically inadequate”). Nonetheless, Descartes didn’t really have a good counterargument for Elisabeth’s challenge.