Philosophical questions that are hard to answer like, what does nothing mean? How can we prove we are or aren't in a simulation? What does consciousness mean?
With the incomprehensible, I've never read his books, but I the closest thing I can think of is Lovecraft. The idea that something is beyond human understanding, or things that merely investigating scares us in a chilling way. Not looking for things that are disturbing, just scary and chilling.
by Dependent-Sherbet-94