Descartes’ cogito, or his famous statement, “I think therefore I am” allows him to assert that he exists because he thinks. The cogito was established before any of Descartes’ later claims in Meditations because it was the first truth he proved after his method of doubt.  It is the first principle he discovers on which all other principles can build. Read more about Descartes' "Meditations" tells you how you exist in this world