PHILOSOPHY:
Descartes poses the idea of an evil demon who decieves us to expose even our most certain beliefs to doubt. If it is possible that there is some being out there fooling us, then we cannot trust our senses.
Descartes finds one point of certainty: that I am a thinking thing.
As a thinking thing, my mind is full of thoughts.
For most of these thoughts, there is no guaruntee that they correspond to anything real.
But we have the idea of God in our minds. Does this idea correspond to anything real? There answer is yes! God is defined as the idea of infinite perfection. I could not come up with this idea on my own. It must have been placed inside me from an outside source. The only possible source adequate to this idea is God himself.
So the idea of God is unique - it must refer to a real being - God.
God as the infinitely perfect being could not deceive us. Goodness is a perfection so God is good and does not deceive. So there is no evil demon deceiving me because God would prevent it. So there is no reason to doubt my basic experiences because they are guaranteed by God.
Does that make sense? (minimum 50 words).