Philosophy

The Method of Doubt is a systematic process of doubting ones own beliefs in order to ascertain which beliefs are actually true. This method was revolutionized in the field of philosophy by Rene Descartes (1596  1650). Descartes employs a basic strategy which aims to defeat skepticism at its own ground. He advocates doubting the fundamental process of reasoning in addition to doubting the evidence of the senses and cultural superstitions. If any specific fact can stand this test, it is truly indubitable and can be considered a solid foundation of knowledge.
In his Meditation 1, Descartes resolves to doubt everything that he believes in order to establish something as being indubitable and entirely certain. He aims to doubt his beliefs, not reject them outright (Lacewing). Descartes tries to establish doubt across three distinct verticals
Perceptual Illusion Descartes believes that the perception about the external world, gained through human senses, may turn out to be mistaken (Philosophy Pages). Things are not always just as they seem at first glance. He lays stress on the fact that it is wise to not fully believe what one perceives. In real life, one can always recognize a mistaken perception by using correct perceptions as a benchmark. Since one cannot be sure of the truth at first, it is possible to doubt every bit of sensory knowledge which is apparent (Kemerling). Here, the question arises - How do we know anything about what it is really like. Descartes argument notes a clear distinction between appearance and reality, laced with a strong suggestion that the world is nothing like how it appears.
The Dream Issue Descartes also describes a systematic method of doubting every perception, by relating to his inability to distinguish his most vivid dreams from real life. In Meditation 1, he says that since his most vivid dreams are indistinguishable from waking experience, he believes that is possible that everything he sees in the world is just a figment of his imagination . On this basis, it is possible to doubt the existence of every physical aspect, including the world itself (Kemerling).
Some philosophers have tried to refute Descartes by claiming that it is definitely possible to differentiate between dream and reality.  But Descartes had shot back saying that we could be dreaming a perfect replica of reality (Lacewing). Other philosophers have argued that if everything were a dream, then it would be impossible to have two separate entities of realism and dream. This seems to have softened Descartes a bit as he modifies his view slightly in his third meditation. Here, he agrees that we can distinguish between dreaming and waking experience, by the greater coherence of perception (Lacewing).

A Deceiving God Finally, Descartes projects an even more radical theory, related to one of the most treasured worldly beliefs. He discusses the consequences of the all-present god turning into a deceiver and rendering all beliefs false. He seems to suggest that it is an evil demon which seems to drive all our experiences. The possibility of the demon could mean that even if an individual is able to distinguish dream from reality, his experiences in real life could be no more real than his dreams (Lacewing). He also brings up a far more terrifying hypothesis that even if all things have been true up until now, the truly omnipotent deceiver could change the world so as to render ones belief false.   
Descartes seems to reconsider this particular idea in his second Meditation. He tries to argue that even if the evil demon exists, there is one thing he can be sure of.  Later, in Meditation 4, he goes about trying to resolve the issues which led him to doubt everything initially. In order to do this, he needed to establish that
There is no evil demon trying to deceive him
He is not dreaming
A physical world including his body does exist
He can trust his senses
The answer to all these questions establishes that God indeed exists and is not a deceiver as Descartes argues in his early Meditations (Burnham and Fieser).
Descartes ideals on certainty and knowledge have come under severe criticism. Many have argued that Descartes sets the knowledge bar really high. If Descartes thinks that for knowledge to be indubitable, one must be unable to doubt it. For this to be true, belief needs to be infallible. However, this level of severity advocated by Descartes can be seen to be abating by Meditation 3, where he feels that he can know whatever is clear and distinct(Lacewing). Descartes ideas on certainty have also come under the hammer as it appears that he is only after the beliefs that he is certain of. However, Descartes refutes this challenge by claiming that people have a habit of drawing conclusions without appropriate evidence. This is where the Method of Doubt could be employed to good use.