The unclear line between ontology and epistemology.

One of my concerns about Descartes is his methodology. Beginning his quest for truth, he seeks answers by, "rejecting as absolutely false everything in which I could imagine the least doubt." If the goal is to simply investigate and establish the fact that one does exists, Descartes is in the right to consider 'I think therefore I am' his first principle. Even though he does this, the line between ontology and epistemology is blurred. What I have trouble agreeing with is the fact that his whole epistemological methodology is based on an ontological claim. Of course, there are some obvious ties to both subjects that make them inseparable. I will not discuss them because my point has to do with only how Descartes thinks of those ties. He writes, "All the things that had ever entered my mind were not more true than the illusions of my dreams." He realizes that he can say something is nonetheless thinking those thoughts. Then, he concludes that, "[it is] a general rule that the things we conceive very clearly and distinctly are all true; but there is only some difficulty in observing well which those are that we conceive distinctly. It does not follow, after realizing that there is some thing that thinks, that one can take empirical sensations as true.

Although I don't agree with the progression of thought here, I do believe his first principle is accurate and that what we conceive very clearly is generally true. The problem is how he leaps from one to the other.