This is not a question about the cogito argument in the closer sense. I will try to isolate my questions from the other questions posted concerning Descartes.
If I understood what I've read correctly, Descartes claims that what he knows clearly and distinctly is true as it can't be any differently. That's true for the cogito, and, let's assume for argument's sake, also for the ergo sum.
Now I read in the Cambridge Companion to Descartes (online available here) that apparently this ergo is no syllogism but an intuition as basic as the cogito itself. My early assumption, namely that Descartes uses logic to infer the ergo sum though he didn't prove logic's truthfulness yet, is thereby rebutted.
But what about inuition? A quote from the Cambridge Companion (p.147):
Second, Descartes' talk of intuition and deduction from intuitions as our two sources of knowledge in the Rules gives way to talk of clear and distinct perception in the Discourse, Meditations, and Principles. He never announces that the faculties are the same, but their equivalence is strongly suggested by the fact that he designates them by similar descriptions: "the light of reason" and "the light of nature." We are told in the Rules that: "intuition is the indubitable conception of a clear and attentive mind which proceeds solely from the light of reason [rationis luce]" (Rule III: AT X 368: CSM 114) and in the Principles that: "the light of nature [lumen naturae] or faculty of knowledge which God gave us can never encompass any object which is not true in so far as it is indeed encompassed by this faculty, that is, in so far as it is clearly and distinctly perceived" (Part I, art.30: AT VIIIA 16: CSM I 203; consider too Meditations: AT VII 38-9:CSM II 26-7).
I already highlighted what I'm aiming at: Even if Descartes doesn't need logic to work before he can actually prove that it does, even if he can "reduce" the ergo sum to a basic intuition - isn't he getting ahead of the argument, relying on the natural light of a non-deceiving god, although it's not before the 3. meditation that he actually proves that there is a god and that he is no deceiver? In other words: If he has not yet proven that it is impossible that the "natural light" we rely on in our knowledge is sent by a deceiving or evil entity, can any step after the cogito be called necessarily true?
To underline the importance of god for knowledge, let me quote one more passage (p.151):
Descartes denies that the atheist has "true knowledge" on the grounds that the atheist is uncertain of whether he is deceived by some god. Prior to proving God's existence and nondeceptive nature, Descartes is just as uncertain as the atheist about the existence of a deceptive god. His clear and distinct perceptions should not produce certainty for him either.
Isn't there at least the possibility that even when we make analytical or deductive judgements, we're mistaken for we're not granted the natural light of the God? Am I missing something? Or is Descartes plainly abandoning his method of "geometrical deduction" and anticipating the proof of God's existence to deduce it?