Descartes was an interesting man. A math genius: founder of analytical geometry.
In philosophy, he's known for Cartesian dualism, the Cartesian demon, the Cartesian Circle, the "Cogito, ergo sum," a version of the ontological argument, his rejection of final causes, his rejection of Scholastic empiricism, a quantifiable definition of matter (in contrast to the Aristotelian-Thomistic qualitative definition), and his methodological doubt.
He's often dubbed the first modern philosopher because he attempted to make a clean break with the past. Make a fresh start.
I'd like to briefly focus on his methodological skepticism. In a couple of respects, there's a Van Tilian aspect to his methodology.
i) In a sense, his methodological doubt is naive. We can't really abstract ourselves from our social conditioning. We can't avoid being influenced by the history of ideas.
However, to give his method a more charitable interpretation, this is an exercise in becoming presuppositionally self-conscious. We have many guiding assumptions which may be so engrained that we're not even aware of them.
It's a useful exercise to take a step back and consider all the things you take for granted without giving it a second thought.
ii) Apropos (i), it's good to consider how many of your beliefs may be unjustified or unjustifiable.
Of course, we can't do that with everything we believe, but it can be useful to do that with important beliefs. After all, many beliefs are questionable.
iii) Conversely, this sifting process can make us more aware of essential beliefs. By process of elimination, what beliefs are indispensable to morality and rationality? Bracketing one or more beliefs, then considering the adjustments that must be made in their absence, is a way of means-testing worldviews.
Likewise, what kind of world must we live in to ground essential beliefs? What other things must be in place to sustain our essential beliefs?
Take the typical Christian apostate. They think they can leave God behind without leaving anything else of consequence behind. Indeed, they think that's an improvement. They are so shortsighted.
It's natural for them to continue believing many things they used to believe as Christians, because these are indispensable beliefs. So they don't stop to consider that atheism commits them to dispensing with these beliefs. They don't consider the intellectual cost of atheism.
iv) Methodological doubt can be taken too far. It's an intellectual exercise. We need to distinguish between paper doubts and real doubts. Just because we have the ability to dream up intellectual traps that we can't escape from doesn't mean doubt is actually warranted in that contrived situation. That reaction confuses imagination with reality.