The answers are: Never accept anything as true that is not recognized clearly and distinctly to be so and divide up problems into as many parts as possible.
Descartes is credited as being the father of modern rationalism. His analytical method to reach certainty revolved around the idea of doubting anything that could be doubted, even, and specially so, sense data. In this way, he reached the conclusion that he could not doubt that he was doubting. From this fact, he built a system of proofs that were supposed to be clear and distinct, that is to say, differentiable and not obscure. His method recommended, also, that one should split problems into its component and smaller parts in order to understand these and how they work together.