A.) Cartesian doubt
Cartesian doubt is a form of methodological skepticism associated with the writings and methodology of René Descartes. Cartesian doubt is also known as Cartesian skepticism, methodic doubt, methodological skepticism, or hyperbolic doubt.
Cartesian doubt is a systematic process of being skeptical about (or doubting) the truth of one's beliefs, which has become a characteristic method in philosophy. This method of doubt was largely popularized in Western philosophy by René Descartes(1596-1650), who sought to doubt the truth of all his beliefs in order to determine which beliefs he could be certain were true.
Methodological skepticism is distinguished from philosophical skepticism in that methodological skepticism is an approach that subjects all knowledge claims to scrutiny with the goal of sorting out true from false claims, whereas philosophical skepticism is an approach that questions the possibility of pure knowledge.
Cartesian doubt is methodological. Its purpose is to use doubt as a route to certain knowledge by finding those things which could not be doubted. The fallibility of sense data in particular is a subject of Cartesian doubt.
There are several interpretations as to the objective of Descartes' skepticism. Prominent among these is a foundationalist account which claims that Descartes' skepticism is aimed at eliminating all belief which it is possible to doubt, thus leaving Descartes with only basic beliefs (also known as foundational beliefs). From these indubitable basic beliefs, Descartes then attempts to derive further knowledge. It's an archetypal and significant example that epitomizes the Continental Rational schools of philosophy.
Descartes' method (broken into four steps including a. accepting only information you know to be true b. breaking down these truths into smaller units c. solving the simple problems first d. making complete lists of further problems)