To make you ask this question! The difficulty to decipher human behaviour is the main subject of this story. To complicate matters further, James chooses as his dubious character a child, and in doing so puts into questions some universal beliefs such as the natural "innocence" of childhood. It also interrogates upon the nature of one's own subjectivity when making a character judgement of another person. Upon what criteria can one base such a judgement, and isn't it rather a question of opinion rather than fact? The credibility of the narrator is never explicitly questioned, but the doubt upon her veracity is nevertheless cast. Her visions or hallucinations of a phantom presence vouch for her own instability, and suggest even the onset of hysteria.
This book is more of a psychological study than a ghost story thriller, but it gave me real goosebumps just the same. In all this uncertainty, the thing we can be sure of is that creating a feeling of doubt, ambiguity and even fear is just what James was after!
Henry James was writing at a time when sexual matters were not explicitly discussed in stories and novels. A female character in a novel might have a dozen children, but there was never any suggestion of how they were conceived. It would appear that James was writing a story about a young boy who was being sexually abused by a man who was given the name Peter Quint, aided and abetted by a woman called Miss Jessel. The boy's eight-year-old sister was apparently not being abused, but she seemed to know what was going on between Quint and her brother. The governess must have suspected the truth, but she was balked by the secrecy of the two children, and she suspected her own sanity. What made the situation worse was that Peter Quint seemed to be tryiing to lure little Miles into a world of the dead. The supernatural fiendishness may have been a substitute for a literal description of a sexual relationship between a man and a boy. It does seem to arouse similar feelings of horror and loathing in the reader. This idea of a child being sexually abused--and further, being sexually abused by a ghost--may represent the extra "turn of the screw" suggested by the narrator at the opening of the story. In other words, the ghostly element was only a substitute, or objective equivalent, for a prohibited description of the literal truth. James was a master of writing suggestively or ambiguously, as we can observe in The Ambassadors, The Jolly Corner, The Beast in the Jungle, The Great Good Place, and other works. Some critics deny that The Turn of the Screw is about pedophilia, but this seems like a legitimate and plausible interpretation which should be open to discussion in our modern era.