Descartes Says, "And Although the Right Conception of This Truth

Descartes says, "And although the right conception of this truth has cost me much close thinking, nevertheless at present I feel not only as assured of it as of what I deem most certain, but I remark further that the certitude of all other truths is so absolutely dependent on it that without this knowledge it is impossible ever to know anything perfectly." In the italicized phrase,
Descartes is referring to the belief in the power of thought.