The Turn of The Screw thoughts!?

For my midterm, we have to answer an essay question. She gave us three or four questions on what it could be and I have been able to come up with several points for each except for the question : how can you account for the ghosts? In other words how can you prove they are real? The only thing I could think of was that she described them perfectly without knowing anything about them but I can't write three paragraphs on just that. What do you think?