He can not doubt that he thinks, therefore he exists.
Descartes cannot doubt his own existence, as captured in his famous statement "Cogito, ergo sum" - "I think, therefore I am."
That he is sentient ie that he thinks.
In Descartes' Meditations, doubt serves as a tool for philosophical inquiry by challenging and ultimately casting skepticism on the reliability of sense perceptions and beliefs. Through methodical doubt, Descartes aims to uncover indubitable truths and establish a foundation of certain knowledge upon which to build his philosophical system. Doubt, therefore, acts as a catalyst for Descartes' quest for certainty and the establishment of foundational principles in his philosophy.
The method of doubt test in Philosophy is a process Descartes uses in order to test his epistemological exploration in his "first meditation" paper. He uses it to investigate the extent of his knowledge and its basis in reasons or experiences. If there is any doubt about a piece of knowledge in Descartes mind, he has to reject it as not true.
When Descartes reflects on his dreams in Meditation I, he uses it to show that sometimes even his own senses deceive him because in dreams what feels like reality is just an illusion. This is significant because it throws into doubt much more of what Descartes thinks he can be sure of and increases his skepticism.
Short answer: Descartes' doubt was like WWI - the doubt to end all doubt. Only by doubting everything could Descartes hope to find anything that was certain (even if the only certainty is that nothing else is certain!).
Descartes' method of systematic doubt has been widely influential in philosophy and science. It has helped to lay the groundwork for modern epistemology by emphasizing the importance of skepticism and rational inquiry. However, its success can be debated, as some critics argue that aspects of Descartes' method are overly radical or unrealistic.
Descartes found it impossible to doubt his own existence. The reason for this was that he felt that thoughts had to come from himself.
Descartes attempts to doubt everything he believes to be true in order to find a foundational belief that cannot be doubted. He starts by doubting his senses, then his perceptions, and finally even the existence of the external world. Through this method of doubt, he arrives at the conclusion that his own existence as a thinking thing ("I think, therefore I am") is indubitable.
Descartes creates the evil genius in an attempt to throw everything that he thinks he knows into question. The evil genius has god-like power that he uses to deceive Descartes. Descartes cannot disprove the existence of such a creature. You cannot trust your thoughts in such a scenario, thus Descartes cannot be certain that any of the information he gets from his senses, his memories, his thoughts, or his beliefs is real.
Descartes' doubt comes to an end when he establishes the existence of a thinking self (cogito), leading him to the conclusion that he cannot doubt his own existence as a thinking being. This realization forms the foundation of his philosophy, emphasizing the certainty of self-awareness as a starting point for knowledge.
The Method of Doubt employed by Descartes requires the individual to place everything he thinks he believes under a close scrutiny. Anything that cannot be proved beyond the slightest shadow of a doubt, Descartes rejects out of hand. Even so simple a perception as holding a piece of paper in your hand, Descartes finds too untrustworthy on which to base his system of philosophy. Can you truly be certain you aren't dreaming, he asks? Or, if not dreaming, what if you were mad, and while you were quite certain within your own mind that you held a piece of paper in your hand, the reality could be nothing of the sort. In this manner, Descartes systematically applies his doubts to every more basic layer of consciousness until all that he is left with is his conviction that if he can doubt, then there must be something that is doing that doubting. He must exist, if he can doubt, although it is unclear as of yet what form that existence may take.