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Elisabeth of Bohemia's objection

If the mind has neither extension nor surface, one cannot see by what end it could push a body, nor how a body could reach it.

Objects to Substance dualism

Elisabeth of Bohemia · Correspondence with Descartes, letter of 6 May 1643 · 1643

1. One thing moves another only by touching it, or by acting on it through a force that has a direction and a point of application: this is how we understand any action, from colliding balls to the signal running along a nerve.
2. But an unextended mind is nowhere, has neither surface nor force: it meets none of the conditions by which we understand one thing to modify another.
3. One would then have to admit an action of an entirely unprecedented kind, of which nothing can be said except that it occurs. That is not explaining the union of soul and body, it is giving it a name.
4. And the puzzle is twofold, for the mind would also have to be reached by the body: a blow, a fever, a glass of wine immediately change what I think and what I feel, as if matter had purchase on what is not matter.

Therefore Dualism saves the mind's irreducibility at the price of its commerce with the body: the purer the soul, the less one understands how it could raise an arm.