The Nature of Evil
Evil, of course, is the primary theme of the book; however, it is not really evil itself, but the nature and interpretation of this evil that is the key theme. There are also several philosophical questions thrown up here; can a house, an inanimate object, be imbued with evil? Can evil spirits possess an environment even if the people living in it are essentially good? Is there really such a thing as malevolence or is it something that we imagine when we are scared? There is definitely an evil presence in the house - the "little stranger" of the title - but each of the characters thinks that it is caused by somebody different. Dr Faraday does not believe in evil at all in the abstract sense, and only believes in a malevolence of spirit caused by a mental condition. Throughout, the evil that is plaguing the family is the main theme, and the nature of it is central to the entire plot of the book.
Science versus Philosophy
A theme that branches off from the key theme of evil is the constant battle between science and philosophy - or in this case, the explained versus the unexplained. We also see how intractable those who believe wholeheartedly in science can be when it comes to explaining a feeling or something that is essentially un-provable. Dr Faraday goes to great lengths to defend the honor of science, even when it is clear he is clutching at straws. He attributes Mrs Ayres' conviction that the spirit in the house is her deceased daughter who is trying to reunite them more quickly to the fact that she is overwhelmed with emotion, using a medical psychology diagnosis to explain her feelings. He says that Catherine's emotional stress is causing her to believe that it is her brother Roderick who is somehow telepathically demonstrating his anger about being locked up in an institution. As the book progresses and it becomes obvious there really is a malevolent force at work in the house, he doggedly refuses to let go of his scientific explanations for everything and blames the noisy plumbing in the house for the sounds that are terrifying the family and the staff.
The British Class System
Class, and the desire to move oneself upwards within the class system, is a theme throughout the book. The Ayres family are aristocratic and envied by many for their outward appearance of wealth and their position in society. It is said that Faraday's mother was once a nursery maid at Hundreds Hall, and he remembers being there as a child to attend a garden fete. Although the riches of long ago have dissipated and the upkeep of the house costs far more than the family are able to afford, their position in society remains unchanged. Faraday feels alternately jealous of the Ayres family for their upper class, and angry with himself for not using his education to move "upwards".
Pessimism About The Future
A Labor Party government has just taken power in England at the time in which the novel is set and this has caused enormous pessimism. In fact, there is such turmoil and fear that people are starting to wonder about their own sanity. There are social changes that suggest that life in general is going to be harder for everyone, and there is a starkness in the way post-war British people feel about the life that is unfolding in front of them.
