The paper examines the role of self-deception in Descartes' Meditations. It claims that although Descartes sees self-deception as the origin of our false judgments, he consciously uses it for his searching for truth. He finds that self-deception is a very productive tool in our searching for truth, since it expands our ability to free ourselves from our old certainties; logical thinking enables us to doubt our certainties but only self-deception enables us to really suspend them.
Descartes, then, proposes a logical-psychological method in first person for philosophical investigation, in which self-deception plays a crucial role. The Cogito should be understood accordingly as a first psychological truth rather than a first philosophical truth. Nevertheless, it is a crucial step in Descartes' philosophical investigation and exposes the relations between the logical aspect and the psychological aspect of philosophical thinking.