essay hw in 3 pages

Answer the question in 3 pages :

Why does Descartes believe that even if the Dreaming Argument undermines our beliefs in the existence of the body and the external world, it does not undermine the truth of mathematical and logical beliefs? How does the evil genius/evil demon argument undermine these latter set of beliefs? What is the one thing that remains certain even if an evil demon exists and is running the world?