Again Smokey, context is everything.

The point with descartes was that he wanted to break from prior philosphical method. The philosphy of his immediate predecessors was based on bold but doubtful premises.

He wanted to apply the type of thought to philosophy that mathematicians applied in Geometry.

His idea was that if he could prove one thing that ws beyond doubt, then he could build an understanding of the universe with that unshakeable truth as his first principle.

It is in the meditations that he attempted to demonstrate how his method of observation and analysis worked, by hopefully succeeding in finding that one doubt free first principle.

He found that while he could doubt everything else, he could not doubt his own existence, as in order to do so, he had to be doing some doubting .. so everytime he eclipsed himself in one place, he would turn around to find himself protected on the other side of the verb.