Besides being antimilitary and asking how madness is defined, the novel explores the problematic nature of identity— in keeping with both the 1960s "identity crises" and the later spread of psychotherapy and self-help books. Literally, the whole plot depends on an identity confusion, hinted at in mysterylike fashion throughout the book, and a deliberate masquerade, revealed more suddenly towards the end. The first scene features an inmate presenting himself as Dr. Fell, the in-house physician, complete with stolen clothing. Moreover, role playing, if not actually switching identities, shows up in Shakespearean drama put on by a mental patient, Reno (with dogs as actors), and in Kane's decision to let the inmates act out the movie The Great Escape, "for therapeutic reasons," even ordering the Marines running Camp 18 to dress as Nazis.

Blatty also plays with names and naming throughout the book. Whether jokingly or through insanity, Cutshaw, the rest of the inmates, and Dr. Fell use other names for Kane so cavalierly that they make the whole concept of identity seem slippery. The names often have symbolic associations, or may just show the disjointed thought of the inmates: P. T. Barnum, Giant Brain, Gregory Peck, and Caribou. Cutshaw's name also slides around, the astronaut being called or calling himself names from Big X to Rumpelstiltskin. Fell's first comment to Kane mixes true insight and absurdity: after calling Kane Vincent (a clue to the protagonist's true identity), he mock-explains that Kane looks like Vincent Van Gogh, "or a lark in a wheat field; I'm not sure which." This theme, played out literally via possession in Blatty's supernatural books, is here both more subtle and basic.

Related to the question of self identity, Blatty offers a fascinating and unusual view of mental illness. Discussing Hamlet, Reno chooses neither traditional interpretation of the character, that Hamlet is insane or that he is pretending. Rather, Reno's theory is that Hamlet is pretending to be crazy so that he does not really go crazy, "and the crazier he acts, the healthier he gets." Acting crazy is a "safety valve" such as Kane suggests that the acting out of The Great Escape may be for his charges, or their imitation madness (if it is imitation) probably is; Kane himself, through most of the book, is engaged in a similar kind of "acting," identity-displacement as a safety valve from truly going insane.

Although The Ninth Configuration contains no supernatural elements, Douglas Winter suggests that it forms a trilogy with Blatty's possession novels, based on the similar themes: "The Exorcist posed the problem of evil; The Ninth Configuration countered by posing a greater mystery, the problem of good; and Legion offered Blatty's solution." The distinctions are not that absolute, if only because Blatty cannot (perhaps no one can) discuss evil or good without considering the other. Still, Winter captures the major emphasis of each book. The movement of The Ninth Configuration is, even more strongly than in the other two novels, from lost questioning to hope. Cutshaw wears a black armband, "mourning the death of God," and badgers Kane for proof of divine help or human good; in his own way, Kane delivers.

Self-sacrifice is featured in all three of these books. Although it is a minor theme in Legion and is strengthened in the film adaptation of The Exorcist III, it forms the core of The Ninth Configuration . In one debate, Cutshaw challenges Kane to give him just one example of a person sacrificing himself to save another. By novel's end, Blatty has delivered two examples: Kane, who gives up much to save Cutshaw from...