Explain why Descartes claims that his knowledge about this cannot be doubted

In Meditation II, Descartes states, "I am; I exist - this is certain." Explain why Descartes claims that his knowledge about this cannot be doubted. Can you think of a refutation, or does it make good sense? Why does he have to prove we have immaterial minds? Keep in mind he also concludes that anything he “clearly and distinctly perceives” is something he cannot doubt (See last paragraphs of Meditation VI).

NOTE: These meditations are found in Rene Descartes' Meditations on First Philosophy in Focus, pages 50-58 and 80-86, respectively. However, reading the whole work is recommended.