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.
That he is sentient ie that he thinks.
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.
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!).
According to Descartes if it is clear and distinct it is real
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
Essentially, Descartes' philosophy was based on the notion of methodical doubt, that is, to doubt absolutely everything that one could not be absolutely certain of. Descartes considered the principal weakness of his predecessor's philosophy to be its subjectivity. He felt that the premises from which previous philosophers deduced truths were not necessarily accurate, and thus did not provide any intellectual certainty. Consequently, Descartes used methodical doubt as the starting point for his philosophy.
He can not doubt that he thinks, therefore he exists.
That he is sentient ie that he thinks.
Descartes' theory of knowledge was to doubt all things and accept as knowledge the things that could not be doubted
Descartes hopes to find out what he can know for certain, without any doubt, about the world and nature.
he believed if you can doubt it, it must not exist.
Descartes hoped to prove with that method of doubt that inconsistencies in research can be proven. By doubting scientific findings, people begin to question it which leads to more research.