What is the main theme of this story?

The Power of the Unknown:

The power of that which is unknown over the human psyche is the thread which ties every other aspect of the novel together. Philip has been raised in such a way that he literally knows nothing about women. His uncle has willingly refused to try to learn about women, arriving at a conclusion uninformed by experience and being happy with it. Philip never knows for sure what the relationship is between Rachel and Rainaldi and is likewise never knows for sure whether his uncle was poisoned to death or whether his mind was poisoned by a tumor. The circumstances of Ambrose Ashley’s demise is the point at which the theme of the unknown transfers to the reader. Aside from the author herself and anyone she may have be personally discussed it with, no reader of the book knows for sure whether Rachel actually killed Ambrose or not, yet almost every reader likely has drawn a conclusion one way or the other. Therein lies the power of the unknown because reader has ever been right, yet most assuredly many have stood unmovable in their own conviction.