That he is sentient ie that he thinks.
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 found it impossible to doubt his own existence. The reason for this was that he felt that thoughts had to come from himself.
Philosophically, Descartes was concerned with the existence of reality.
he believed if you can doubt it, it must not exist.
For Descartes, this was the fact that he existed as a thinker. He was perfectly sure he was thinking; and he was likewise convinced that to be thinking he had to exist as some kind of substance.
Descartes cannot doubt his own existence, as captured in his famous statement "Cogito, ergo sum" - "I think, therefore I am."
He can not doubt that he thinks, therefore he exists.
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 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 hopes to find out what he can know for certain, without any doubt, about the world and nature.
René Descartes, a French philosopher, created the system of systematic doubt. In his work "Meditations on First Philosophy," Descartes employs the method of doubt to question all his beliefs in order to find a foundation of certainty in knowledge.
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.
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 believed that doubt was a crucial tool in the search for truth. By subjecting all his beliefs to doubt, he aimed to find a foundation of knowledge that could not be doubted. This led him to his famous statement "Cogito, ergo sum" (I think, therefore I am), which served as the starting point for his philosophical system.
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.
Systematic doubt. Descartes could doubt everything except for one thing - his own existence. Cogito ergo sum - "I think, therefore I am" - became his first principle.