“It is now some years since I detected how many were the false beliefs that I had from my earliest youth admitted as true, and how doubtful was everything I had since constructed on this basis; and from that time I was convinced that I must once for all seriously undertake to rid myself of all the opinions which I had formerly accepted, and commence to build anew from the foundation, if I wanted to establish any firm and permanent structure in the sciences.”

Descartes lays out his primary method of investigation. He informs the reader that, to arrive at truth, he must doubt his previous assumptions. This doubting is necessary as the previous assumptions have created a faulty base for the sciences. 

“Now let us assume that we are asleep and that all these particulars, e.g. that we open our eyes, shake our head, extend our hands, and so on, are but false delusions; and let us reflect that possibly neither our hands nor our whole body are such as they appear to be.”

Descartes, to justify his doubt, addresses the possibility that he may be dreaming. The dream argument posits that one cannot differentiate between wakefulness and sleep, as in dreams one may also feel pain or emotions.

“I shall then suppose, not that God is supremely good and the fountain of truth, but some evil genius not less powerful than deceitful, has employed his whole energies in deceiving me.”

In another attempt to justify his doubting of all beliefs, Descartes moves to God. He hypothesizes that God is an evil genius who deceives the subject. This supposition allows him to call everything into question.