First, Thank you Sparknotes for posting an uncompleted note TWICE.
Let's try this again.
The themes of insanity can be interpreted in many ways in this novel.
All of the themes I am about to discuss have to do with the Governess.
1.) The Governess is possibly absolutely sane throughout -
~This theory is the one I feel is the least likely option in the novel. With everything going on, if she wasn't already insane before, the events in the story should have driven her to insanity.
2.) The Governess is driven crazy by the events in the novel.
~In the last scene of the novel, she lost it all. She see's Quint for the last time and Miles falls dead in her arms. Either seeing the Ghosts so much or having the child die in her arms drove her to complete insanity.
Also, it is possible that her insanity caused her to murder the young boy accidentally.
3.) The Governess hallucinates some things.
~It is possible that she was some-what insane before she met the children. She hallucinated the ghost encounters as well as some of the conversations with the house keeper, whom she seems to have a slightly homoerotic relationship with. She also may or may not have Hallucinated Miles and/or killed him.
4.) The Governess hallucinated everything:
~The entire story was a hallucination and is just passed on later. She never realizes her insanity.
Consider these options when thinking about the book.