“Regan had the physical syndrome of possession. That much he knew. Of that he had no doubt. For in case after case, irrespective of geography or period of history, the symptoms of possession were substantially constant. Some Regan had not evidenced as yet: stigmata; the desire for repugnant foods; the insensitivity to pain; the frequent loud and irrepressible hiccuping. But the others she had manifest clearly: the involuntary motor excitement; foul breath; furred tongue; the wasting away of the frame; the distended stomach; the irritations of the skin and mucous membrane. And most significantly present were the basic symptoms of the hard core of cases which Oesterreich had characterized as genuine possession: the striking change in the voice and the features, plus the manifestation of a new personality.”

“...the exorcist should not believe too readily that a person is possessed by an evil spirit; but he aught to ascertain by the signs by which a person possessed can be distinguished from one who is suffering from some illness; especially one of a psychological nature. Signs of possession may be the following: ability to speak with some facility in a strange language or to understand it when spoken by another; the facility of divulging future and hidden events; display of powers shich are beyond the subject's age and natural condition; and various other conditions which, when taken together as a whole, build up the evidence.”

“Yet I think the demon's target is not the possessed; it is us . . . the observers . . . every person in this house. And I think---I think the point is to make us despair; to reject our own humanity, Damien: to see ourselves as ultimately bestial; as ultimately vile and putrescent; without dignity; ugly; unworthy.”

“Every man that ever lived craved perfect happiness, the detective poignantly reflected. But how can we have it when we know we’re going to die? Each joy was clouded by the knowledge it would end. And so nature had implanted in us a desire for something unattainable? No. It couldn’t be. It makes no sense. Every other striving implanted by nature had a corresponding object that wasn’t a phantom. Why this exception? the detective reasoned. It was nature making hunger when there wasn’t any food. We continue. We go on. Thus death proved life.”

“But I'll tell you something, Father; you give me Regan's identical twin: same face, same voice, same smell, same everything down to the way she dots her i's, and still I'd know in a second that it wasn't really her! I'd know it! I'd know it in my gut and I'm telling you I know that thing upstairs is not my daughter! I know it! I know!”

“We use concepts like "consciousness"---"mind"---"personality," but we don't really know yet what these things are.' He was shaking his head. 'Not really. Not at all.”

“From the cab stepped a tall old man. Black raincoat and hat and a battered valise. He paid the driver, then turned and stood motionless, staring at the house. The cab pulled away and rounded the corner of Thirty-sixty Street. Kinderman quickly pulled out to follow. As he turned the corner, he noticed that the tall old man hadn't moved but was standing under the streetlight glow, in mist, like a melancholy traveler frozen in time.”