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Full Idea
I observe two ultimate classes of things: intellectual or thinking things, pertaining to the mind or to thinking substance, and material things, pertaining to extended substance or to body.
Gist of Idea
There are two ultimate classes of existence: thinking substance and extended substance
Source
René Descartes (Principles of Philosophy [1646], I.48)
Book Ref
Descartes,René: 'Philosophical Essays and Correspondence', ed/tr. Ariew,Roger [Hackett 2000], p.243
A Reaction
This is clear confirmation that Descartes believed the mind is a substance, rather than an insubstantial world of thinking. It leaves open the possibility of a different theory: that mind is not a substance, but is a Platonic adjunct to reality.