Abstract

Descartes' methodical doubt is being criticized by naïve realists and others who don't find doubt as a good starting point for metaphysical thought, however, the philosophical achievements of his method have been absorbed in all later philosophies. The objective of this paper is to demonstrate how an inevitable question concerning the foundation of Descartes' mathesis universalis, which led him to investigate this foundation by applying this very method in Metaphysics, has finally enabled him to discover his most important philosophical principle, i.e., the necessary relation between being and reason in the identity of the "I". To demonstrate this, after summarizing the fundamental ideas of mathesis uneversalis, and showing the inner necessity of introducing the methodical doubt to fortify these ideas, the decisive role of metaphysical doubt as the third step of the methodical doubt, and malicious demon as its second phase would be highighted. It would also become clear that the metaphysical doubt has a great importance as a turning point from different aspects: as a knot that ties mathesis universalis with its underlying Metaphysics; as a bridge from within the psychological "I" to the metaphysical "I".