To use a philosophical term, the paranoiac is the Simon-pure "solipsist."
1871, coined from Latin solus "alone" (see sole (adj.)) + ipse "self." The view or theory that self is the only object of real knowledge or the only thing that is real. "The identification of one's self with the Absolute is not generally intended, but the denial of there being really anybody else" [Century Dictionary].
The belief that all reality is just one's own imagining of reality, and that one's self is the only thing that exists.