Order of reasons on one hand, meditative exercise on the other, are the two constant registers of Descartes’ metaphysical enterprise. Keeping both in sight, this book explores some argumentative strategies by which Descartes implements his philosophical challenge to his materialist and atheist interlocutors, by isolating and putting in the "right" order the basic metaphysical notions of ego, God and matter and their complex relationship. In particular, the research sheds new light on Descartes’ concept of metaphysical experience, and reconsiders the relation between his metaphysical itinerary and his scientific project.