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Full Idea
Because each one of us understands what he thinks, and that in thinking he can shut himself off from every other substance, we may conclude that each of us is really distinct from every other thinking substance and from corporeal substance.
Gist of Idea
In thinking we shut ourselves off from other substances, showing our identity and separateness
Source
René Descartes (Principles of Philosophy [1646], I.60)
Book Ref
Descartes,René: 'Philosophical Essays and Correspondence', ed/tr. Ariew,Roger [Hackett 2000], p.247
A Reaction
This seems to be a novel argument which requires elucidation. I can 'shut myself off from every other substance'? If I shut myself off from thinking about food, does that mean hunger is not part of me? Or convince yourself that you don't have a brother?