"he might have had a monomania on the subject of his departed idol"
Monomania is a psychiatric condition in which the sufferer focuses on only one idea or type of idea. Heathcliff's monomania is his obsession with Cathy and his desire to be connected with her after death. According to Graeme Tytler, who looked at the book in terms of 19th century psychological theory, much of Heathcliff's behaviour in the later part of the book can be attributed to monomania, including his being haunted by Catherine's image and talking to her ghost. A psychological interpretation of the book can be found here