Cartesian clear and distinct ideas simply don't work in philosophy at all and have proved to be a complete failure.
This development in thought, does however, show us an important point in the development of modern thought. Cartesian thought succeeded in narrowing the concept of knowledge down to only one particular type of knowledge. Specifically scientific or perhaps more accurately mathematical knowledge.
The only ideas that CAN be reduced to the kind of clear distinct certainties that Descartes wanted are mathematical or quantitative ideas. This ultimately lead to the notion that reality is only quantitative in nature, in other words, materialism. Descartes, of course, never intended that, but that is the problem with ideas, they often have unintended consequences.
The triumph of Cartesian epistemology lead to the reduction of knowing down to only mathematical knowing.
Once that happens basically everything of value in philosophy is lost. If all valid knowledge is only quantitative knowledge then things like justice, goodness, truth, love, hope, beauty, are all no longer valid knowledge. Wisdom itself basically becomes a non-entity. What is the point of philosophy that can't lead you to wisdom, or goodness, or love, or hope etc.
The topic of epistemology is important as well because it illustrates on of the major phases of the inevitable decline of Modern thought.
Modern thought basically began with Metaphysical skepticism. This occurred when Aristotelian and Platonic Realism was abandoned in favor of Ockhamist Nominalism.
This fundamentally changed how objects were viewed, and it forced philosophy to re-evaluate how we can know the true nature of any object.
Cartesian thought is an example of one of the ways that subsequent modern thinkers tried to prove that we could in fact know objects. Classical Metaphysical Realism allowed philosophers to begin from objects in the world and ask "how can this thing exist and how can we know the thing as it is in itself."
When that was lost as a result of Nominalism, philosophers found themselves regressing and having to find a new starting point. They could not longer start with the statement "this thing exists, what can I know about it." They eventually found themselves having to ask "can I prove that this thing exists, before I even ask if I can know anything about it?"
The only starting point left to them after the destruction of Classical Metaphysics was the mind itself. So they had to try and solve the dilemma "How do I reason from my own mind to the world existing, and then to me being able to know things in the world?"
This is how Descartes found himself in the position of having to prove that he himself existed before he could try to progress any further in philosophy.
Modern thinkers essentially found that it was basically impossible to progress beyond the mind to the objective world. There was simply no way to reason from "My mind exists" to "the objective world exists and is knowable", in the Cartesian sense of clear and distinct, certain ideas.
This lead to the next major phase of Modern thought, which was the beginning of the end of Modern thought and the beginning of Post-Modern thought. It was the Kantian revolution. More specifically it was Epistemological skepticism.
At that point we had gone from saying there are no real universal entities to saying, it is functionally impossible to know individual entities objectively within the world.
We are in the final phase now, which is linguistic skepticism. We went from there are no real universal entities, to individual entities cannot be objectively known, to language cannot express objective knowledge.
Another way of saying all that is, there is no universal truth, individual truth cannot be known, and truth cannot be communicated with language.
Virtually the whole of the development of modern thought consists of philosophers sawing off the branch on which they are sitting.
All epistemology and knowledge rests upon faith. Without faith, it is literally impossible for knowledge to exist.