What can I know for certain?

The evil genius is a ground of doubt for most of my beliefs.

The evil genius may make me believe things that are false.

But it cannot be that the evil genius makes me believe that I exist if it is false that I exist. (I must exist in order to believe, or doubt, anything.)

So, the evil genius is not a ground of doubt for my belief that I exist.

Descartes:	"Cogito, ergo sum."
	"I think, therefore I am."

"I exist" is thus certain and indubitable.

This ("I exist") is thus the "Archimedian point" Descartes sought.