Doubt and Contradiction

Descartes' assertion, in Principle VII, that 'I am' is the "first and most certain" of all propositions seems to overlook the prior establishment of 'I think', via the implicit argument, 1. I cannot doubt that I am doubting; 2. Therefore, 'I doubt' is certain; and 3. Doubting is a species of Thinking.  Regardless, textually notable in the passage is the invocation of the Law of Contradiction, which does not explicitly appear at comparable stages of either the Discourse or the Meditations.  However, the reference is also a reminder that he has failed to vet the Law--e. g. on the grounds that God may be deceiving him--which he could explore with at least as much justification as submitting Mathematics to the test.  So, one diagnosis of this neglect is that his concept of Will, in either its human or its divine mode, is not absolutely 'free', i. e. because it is constrained by Logic.  Another is that recognizes that without the Law, he would be trapped in a Skepticism, with the establishment of 'I think', 'I am', and 'God exists', thus, a lot more difficult, at best.