In Meditation One Descartes is worried about what he can know. What he cite as the primary source of what he knows or perhaps what he thinks he

In Meditation One Descartes is worried about what he can know. What he cite as the primary source of what he knows or perhaps what he thinks he knows? Is this a reliable source? Why are why not? In Meditation VI How does Descartes propose to justify our judgments about the external world?