If there were demons, then perhaps there were angels

An authorized account of how William Peter Blatty came to write both the novel and the screenplay of The Exorcist. Excited by an article he read as a student, Blatty describes the work he put into researching his novel and the frustractions he encountered in turning the book into the screenplay. Blatty's fascination with the subject was born out of an interest in showing that the existence of God, in contemporary America, could be proved, as he says, If there were demons, then perhaps there were angels and probably God and a life everlasting. His research eventually led him to tracking down priests who had participated in actual exorcisms. In doing so, Batty gathered enough evidence to write a novel that incorporated many of his factual findings.