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.
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 hopes to find out what he can know for certain, without any doubt, about the world and nature.
Descartes' radical doubt refers to his method of systematically doubting all beliefs and knowledge in order to establish a foundation of certainty upon which to build his philosophy. By doubting everything that can be doubted, Descartes aimed to arrive at indubitable truths that could serve as a solid basis for his philosophical system.
Descartes believes in looking for the truth to answers. In order to do this, doubt must be used until the conclusion is reached.
Descartes found it impossible to doubt his own existence. The reason for this was that he felt that thoughts had to come from himself.
see
Descartes' method of doubt was a systematic approach to examine the foundations of knowledge by doubting everything that could possibly be doubted. He aimed to find a firm foundation for knowledge by doubting even his own existence, ultimately arriving at his famous conclusion, "Cogito, ergo sum" (I think, therefore I am). Through this method, Descartes sought to establish a new epistemological foundation based on indubitable truths.
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.
Philosophically, Descartes was concerned with the existence of reality.
Descartes' theory of knowledge was to doubt all things and accept as knowledge the things that could not be doubted
he believed if you can doubt it, it must not exist.