Described as an eminent "physicist, ballistician, philosopher and psychologist", de Selby is known for his paradoxically non-scientific beliefs and personality. De Selby serves as an unseen character in The Third Policeman, where he is discussed at length in references and footnotes which tenuously link his unorthodox theories and areas of research to the plot. (In one footnote, he attempts to dilute water; in another, he posits that night is caused by the accumulation of "black air".) O'Brien's novel The Dalkey Archive focused on de Selby himself.