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 found it impossible to doubt his own existence. The reason for this was that he felt that thoughts had to come from himself.
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!).
That he is sentient ie that he thinks.
he believed if you can doubt it, it must not exist.
i am i exist
Descartes found it impossible to doubt his own existence. The reason for this was that he felt that thoughts had to come from himself.
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.
see
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.
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.
The one thing that Descartes couldn't doubt was his thoughts, he thinks there for he is. To be a good philosopher, you need to be skeptical (to ask a lot of questions), Descartes asked questions then doubted them. That brought in dualism. He is considered one of the greatest thinkers of the enlightenment period.
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.