Being a foundationalist, Descartes needs to destroy the foundations of his beliefs so that in his Meditations he will be able to build upon new foundations of undeniable and self evident truths. In order to do this Descartes must first find a valid argument that will allow him to doubt his foundation beliefs and in turn doubt what is considered to be reality. He begins by first noting that one cannot trust their own senses, because we can be deceived by our sciences. An example of such would be if one looks at an optical illusion, they are seeing something that is not really there, and therefore are being deceived by their sense of sight. But this is not enough to justify doubting all things, so Descartes offers a different approach, the Dream Argument.
The Dream Argument is essential in because it allows one to logically question not only the senses but their surroundings and actions as well. 
Although one can doubt that what they see or hear is not really as is perceived; a person cannot deny that they are for instance, standing, thinking about how their senses are deceiving them, with their feet planted on the ground, in their bedroom, feeling a little tired and so on. Only if one was, as Descartes writes, “..insane, whose brains are impaired by such an unrelenting vapor of black bile..” that they believe they are something other than what they are, would one doubt reality, without an argument. The argument is as follows: If the experience of a dream is indistinguishable between that dream and reality; and there is no test to differentiate between dreaming and awakens, then one must doubt the world outside their minds. This is so because even if one believes they are awake and perceiving their surroundings soundly, they have no way of knowing for certain that they are not, at that moment, dreaming. Still this argument is not sufficient in Descartes’ quest to doubt “everything”. This is so because even when we are dreaming we still know certain undeniable truths or a priori knowledge, these are facts such as a5 + b5 = c5 (in a right triangle) or that triangles have three sides.
Descartes then begins to entertain the idea of a God who created all things, could be deceiving us so that we were wrong in our thinking when we believe a priori beliefs. But Descartes believes God to be all good, and being so, God would not deceive us or even allow us to be so deceived. Descartes cuts short this argument, and explains that he believes in God, as he writes “…not out of frivolity or lack of forethought, but for valid and considered reasons.”. Descartes therefore abandons the thought of God as he is thought of by most people (for the time being) and instead offers a different “deity”. In order to be able to doubt even a priori knowledge Descartes comes up with what is known as the Evil Genius Hypothesis. The Evil Genius Hypothesis is this: instead of a “supremely good God, the source of truth” there exists an Evil Genius who deceives completely so that all that is perceived is not actually what exists in the “real” world. Because the Evil Genius can mislead a person into thinking for instance, that a triangle has four sides or some other false belief, there is no realm of perception that is not touched by the argument. The Evil Genius argument allows Descartes to logically distrust and doubt a priori and a posterior knowledge. This argument completes Descartes’ mission for a logical argument that will allow him to doubt everything, and start from the bottom of the hierarchy of his beliefs.
