  The famous Cartesian hypothesis is of a demon who deceives me in all of my beliefs about the external world, while also ensuring that my beliefs are completely coherent. This possibility cannot be ruled out by any experiences or beliefs, because of how the deceiving demon is defined. This hypothesis is also contrary to my beliefs about the lake.  So my beliefs about the lake are not justified, according to the above principle. And there is nothing special about my beliefs about the lake. Everything I believe about the external world is incompatible with the deceiving demon hypothesis. Skeptics conclude that no such belief is justified.