Descartes, Doubting, Thinking

As has been previously discussed, Descartes finds Certainty in his uncertainty, i. e. he cannot doubt that he doubts. What rescues him from Skepticism is his further proposition that Doubting is Thinking. That proposition can mean one of two things: either that Doubting and Thinking are identical, or, that the former is a species of the latter. That he means that Doubting is a species of Thinking seems most likely, because to reduce other modes of Thinking to Doubting seems difficult. But, for that very reason, his inference from "'I doubt' is certain" to "'I think' is certain" is invalid, i. e. it entails an illegitimate generalization of a property of Doubting to all modes of Thinking. Now, such a clumsy introduction of Thinking into his argument may represent more than a relaxation of logical rigor. It encourages the suspicion that Descartes' primary aim is not a presupposition-less reconstruction of Knowledge, but a re-grounding of traditional soul-body dualism.