Whenever I Think, Then I Am

According to Propositional Logic, 'A, therefore B' is equivalent to 'If A, then B', and to 'Whenever A, then B'.  Furthermore, according to Modal Logic, 'necessarily (If A, then B)' is not equivalent to 'If A, then necessarily B'.  Now, if it is granted that Descartes demonstrates: 1. Whenever I doubt, I am thinking; 2. Whenever I think, I exist; 3. Whenever I think, God exists; and 4. Whenever I exist, God exists, it still does not follow that 5. It is certain that I think; 6. It is certain that I exist; or 7. It is certain that God exists.  So, despite his expression of respect for the Law of Contradiction in Principle VII, his presumption that he has demonstrated #5, #6, or #7 on the basis of 'I can doubt my sense experience', is based on loose Logic.