EXORCISM. The English word exorcism derives from the Greek exorkizein, a compound of ex (out) plus horkizein (to cause to swear, or to bind by an oath). Whereas in Greek the word sometimes is used simply as a more intensive form of the root, meaning "to adjure," English derivatives usually designate a "swearing out" of invasive spiritual forces from the body in a formal rite of expulsion. Thus exorcism cannot fully be understood without reference to the concept of spirit possession, the state that it redresses.

The spirits to be exorcised most commonly are conceived either as demons or as restless ghosts. These evil spirits penetrate into the bodies of their victims and completely control, or at least strongly influence, their actions. Possessing spirits may also cause physical illness by interfering with the body's normal physiological processes or mental illness by affecting the will, intellect, and emotions. Yet...