Descartes believes in looking for the truth to answers. In order to do this, doubt must be used until the conclusion is reached.
According to Descartes if it is clear and distinct it is real
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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.
Descartes' evil genus argument is about a creature with god-like power that can deceive Descartes regarding absolutely anything. The purpose of conceptualizing such a creature is to cast everything you think you know into doubt so that you might see what remains.
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.
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 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.
Descartes' evil genus argument is about a creature with god-like power that can deceive Descartes regarding absolutely anything. The purpose of conceptualizing such a creature is to cast everything you think you know into doubt so that you might see what remains.
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.