His job is to identify the badspirits that are attaching themselves to the juus (soul) of a person, and by placing himself in a trance he is able to purify the afflicted soul.
Not long after embarking upon the subject of God and melancholy, for example, Mr. Burton veers into "A Digression of the Nature of Spirits, Bad Angels, or Devils, " which leads into an elaborate taxonomy of evil creatures, including the question of whether some devils "have excrements" or "feel pain if they are hurt, " and whether there can be both good and bad devils.