Mulligan repeats a fellow medical student's surmise that Stephen has "General paralysis of the insane." Gifford notes that the actual label was “general paresis of the insane,” a “genteel medical term for syphilis of the central nervous system,” but he observes that medical students sometimes used g.p.i. as joking slang for mere eccentricity.
Arguments have been made, however—notably by Kathleen Ferris—that Joyce did suffer from paresis. The mental illness of his daughter Lucia was reportedly diagnosed as schizophrenia in the 1930s, and Ferris acknowledges that that condition could account for some of her symptoms. But syphilitic paresis accounts for more, she suggests, and can be caused by spirochetes transmitted in utero (106-11). Lucia once told her father “that she had syphilis” (111). Ferris sees the evidence of this disease in Joyce's fictional offspring as well: Stephen and Bloom, she argues, between them display an assortment of symptoms (stiff walk, bad eyesight, digestive problems, hallucinations) that might be caused by syphilis.
Syphilis aside, Joyce’s works contain a number of instances of mental imbalance. The mother in Eveline (a story in Dubliners) lived a “life of commonplace sacrifices closing in final craziness,” and the old man in An Encounter seems deeply disturbed. Walking down a lane in A Portrait, Stephen hears “a mad nun screeching in the nuns’ madhouse beyond the wall. — Jesus! O Jesus! Jesus!” Ulysses introduces two disturbed individuals in Lestrygonians: Dennis Breen, and Cashel Boyle O’Connor Fitzmaurice Tisdall Farrell. When Bloom points out the “dotty” Farrell to Breen’s wife, she replies, “Denis will be like that one of these days.” Haines, with his predatory nightmares and itchy trigger finger, is "A woful lunatic!" Even the even-tempered Bloom fears slipping into insanity, perhaps because of his father's suicide. In Ithaca, we learn that he fears "The committal of homicide or suicide during sleep by an aberration of the light of reason."