Descartes' work is a work of thought, which he wanted and built with method, strictness and constancy his whole life through. Thanks to this work, we do not discover as much the truth of the things as the way we can build our own thought and system of convictions. And the method Descartes is suggesting and applying in the mean time is aimed at coherence. To think truly the world is consequently the most reliable way to think with coherence.The subject of the study is to highlight the rules that indeed make up what we may call the Cartesian maxim of the thought. With this aim in view, we start with the description of what is for Descartes the thought on one hand, and on the other hand, this world which is precisely to be thought. We are then led to examining and criticizing the role of God's existence in the Descartes' system, which aims to guarantee the coherence of his own thought. However, in spite of the application of his maxim, Descartes did not avoid an error which was to weaken his system of convictions, that is to say his conception of the matter as pure expanse. Even if the "novel of nature" he then imagined on this erroneous basis is coherent, it unfortunately does not represent the reality of the world. By introducing inter-subjectivity in the ethics of the thought, i.e. by accepting to rub our own system of convictions to the one of the "Other", whose existence and value we recognise, we no doubt can avoid this kind of drift. So we establish that the idea of Man, who is both the Cartesian ego and this Other one, takes with great benefit the place of the idea of God, in the prospect of guaranteeing the truth.
