Descartes' conceptual distinction (or distinctio rationis) is commonly understood to be a distinction created by the mind's activity without a foundation in re. This paper challenges this understanding partially based on a letter to an unknown correspondent in which Descartes claims not to admit distinctions without a foundation. He goes on to claim that his conceptual distinction is not a distinctio rationis ratiocinantis (i.e. a distinction of reasoning reason) but is something like a formal distinction or, more precisely, a distinctio ! rationis ratiocinatae (i.e. a distinction of reasoned reason). This remark is then explored through Descartes' other writings and his scholastic intellectual heritage. I conclude that Descartes' conceptual distinction just is a formal distinction understood as a distinctio rationis ratiocinatae as expressed in the works of Scotus, Suarez and Eustachius, and so it has a foundation in re in very much the same way as a formal distinction.

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