... Descartes means by the reason is that the reason attempts what is universal and what is necessary. Moreover, Descartes believes that in the jungle of unreliable knowledge, we have to create a " First Philosophy" to know foundations and principles under it . Thus, everything has a single ground and single foundation. This reasoning leads Descartes to think that he should reject all his opinions and he needs to find in each of them at least some reasoning for doubt. Descartes says that in the first sentence of the quotation that " I am certain that I am a thinking thing." He uses the certainty here by referring to himself because he thinks that every thinking thing who is certain of himself is certain. Descartes has methods for determining certainty which are erasing everything that we know and finding a foundation that our experience can rely on because we ...

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