Full Idea
Descartes himself was well aware of the interaction problem, and corresponded uncomfortably with Princess Elizabeth on the matter; …he pointed out that gravity is causal despite not being a physical object.
Gist of Idea
Descartes discussed the interaction problem, and compared it with gravity
Source
report of René Descartes (works [1643]) by William Lycan - Consciousness n1.3
Book Reference
Lycan,William G.: 'Consciousness' [MIT 1995], p.131
A Reaction
Lycan observes that at least gravity is in space-time, unlike the Cartesian mind. Pierre Gassendi had pointed out the problem to Descartes in the Fifth Objection to the 'Meditations' (see Idea 3400).
Related Idea
Idea 3400 Things must have parts to intermingle [Gassendi]