I Think, Therefore God Exists

Descartes' famous phrase, 'I think, therefore I am', appears in the Discourse on Method, but not in the Meditations, not adventitiously, but because it is a stage in the former, but not in the latter.  In the Discourse, 'I think', via the irreducibility of the application of the method of Doubting, is the first certainty at which he arrives., from which 'I am' follows.  In contrast, in the Meditations, the axiom is 'I exist', derived from 'An evil God may be deceiving me', via the irreducibility of the deceived 'me'.  From there in the Meditaitons, he next proves that 'I am a thinking being', which supplies him with the proposition 'I have a thought of God', which serves as the premise of both the Third and Fifth Meditations, in proofs of the existence of God.  Furthermore, the topic of the Fourth Meditation--"Of the True and the False", extends the one of the Theological concepts--Perfection--of the Third, i. e. in establishing that because God is perfect, and deception involves imperfection, God is not a deceiver..  So, while the staple in academic curricula is the more explicitly Theological of Descartes' two mature works, his greater influence, not only on the popular image of him, but in the primacy of Epistemological methodology--Empiricism, Rationalism, Phenomenalism, Transcendentalism, Phenomenology, etc.--in subsequent Modern Philosophy, stems from the other book.  In other words, the Discourse presents his Philosophical innovation, while the Meditations merely continues the Medieval tradition of subordinating Philosophical methods to Theological purposes, i. e. its primary theme can be expressed as 'I think, therefore God exists'.