This evil demon thought experiment is Descartes’ way of pushing doubt to its limits. If there was one thing that we could be sure the evil > demon couldn’t trick us about, that would be amazing. It would also > provide a way of answering those people who claim that we can’t know anything at all for certain.
[A little history of philosophy, Nigel Warburton, Chapter XI]
What does the word "that" refer to? Is it the thing that we could be sure the evil demon couldn’t trick us about?