Literacy
Throughout the novel, reading and literacy are shown to be sources of both power and pleasure. Heathcliff purposely keeps Hareton uneducated as a way to control the young man and to get revenge on Hareton's father, Hindley. Likewise, Cathy gives books to her servant, Michael, to convince him to deliver her love letters to Linton. The graffiti at Wuthering Heights at the beginning of the novel also serves as a kind of dominion; by carving their names into the wall, Catherine Earnshaw and her daughter ensure that their spirits will always preside over the crumbling house. However, the characters also derive significant pleasure from reading; it is one of Cathy's few solaces during her miserable first months at Wuthering Heights, and it eventually serves as a pretext for her to bond with Hareton.
Solitude
For a novel that draws its plot from the vicissitudes of interpersonal relationships, it is notable how many of the characters seem to enjoy solitude. Heathcliff and Hindley both state their preference for isolation early in the novel, and Lockwood explains that solitude is one of the reasons he chose to move to the remote Thrushcross Grange. Each of these characters believes that solitude will help them get over romantic disappointments: Heathcliff becomes increasingly withdrawn after Catherine's death; Hindley becomes crueler than ever to others after he loses his wife, Frances; and Lockwood's move to the Grange was precipitated by a briefly mentioned romantic disappointment of his own. However, Brontë ultimately casts doubt on solitude's ability to heal psychic wounds. Heathcliff's yearning for Catherine causes him to behave like a monster to people around him; Hindley dies alone as an impoverished alcoholic; and Lockwood quickly gives up on the Grange's restorative potential and moves to London.
Doubles
Given the symmetrical structure of Wuthering Heights, it follows naturally that Brontë should thematize doubles and doubleness. Catherine Earnshaw notes her own "double character" (66) when she tries to explain her attraction to both Edgar and Heathcliff, and their shared name suggests that Cathy Linton is, in some ways, a double for her mother. There are also many parallel pairings throughout the novel that suggests that certain characters are doubles of each other: Heathcliff and Catherine, Edgar and Isabella, Hareton and Cathy, and even Hindley and Ellen (consider the latter's deep grief when Hindley dies, and that they are 'milk siblings'). Catherine's famous insistence that "I am Heathcliff" (82) reinforces the concept that individuals can share an identity.
Self-knowledge
Brontë frequently dissociates the self from the consciousness––that is, characters have to get to know themselves just as they would another person. This becomes a major concern when Catherine Earnshaw decides against her better judgment to marry Edgar Linton; she is self-aware enough to acknowledge that she has a 'double character' and that Heathcliff may be a better match for her, but she lacks the confidence to act on this intuition. Self-knowledge also affects how characters get to know others; Isabella knows how violent Heathcliff is, but is unable to acknowledge this because she believes herself capable of controlling him.
Disease and contagion
Disease and contagion––specifically consumption, or as it's known today, tuberculosis––are inescapable presences in Wuthering Heights. Isabella becomes sick after meeting Heathcliff, and Catherine Earnshaw indirectly kills Mr. and Mrs. Linton by giving them her fever. Even emotional troubles are pathologized much like physical illnesses; consider how Catherine's unhappy marriage and Heathcliff's return contribute to the 'brain fever' that leads to her death. Perhaps most importantly, Lockwood falling ill is what motivates Ellen to tell the story in the first place. The prominence of disease in the novel is a physical indicator of the outsize influence that individuals have on each other in Brontë's world––getting too close to the wrong person can literally lead to death.
Sibling relationships
Sibling relationships are unusually strong in the Earnshaw and Linton families. Indeed, the novel's most prominent relationship––the love between Catherine and Heathcliff––begins when the two are raised as siblings at Wuthering Heights. It is never entirely clear whether their love for each other is romantic or the love of extremely close siblings; although Catherine expresses a desire to marry Heathcliff, they are never shown having sex and their union seems more spiritual than physical. After Catherine's death, Heathcliff gets revenge on Edgar for marrying Catherine by encouraging Isabella to marry him and then mistreating her. Given that Emily Brontë is thought to have had no friends outside of her own family (although she was very close to her brother Branwell and her sisters Anne and Charlotte), it is perhaps unsurprising that close sibling relationships are a driving force in her only novel.
Humanity versus nature
Brontë is preoccupied with the opposition between human civilization and nature. This is represented figuratively in her descriptions of the moors, but she also ties this conflict to specific characters. For example, Catherine and Heathcliff resolve to grow up "as rude as savages" (46) in response to Hindley's abuse, and Ellen likens Hindley to a "wild-beast" (73). The natural world is frequently associated with evil and reckless passion; when Brontë describes a character as 'wild,' that character is usually cruel and inconsiderate––take for example Heathcliff, Catherine Earnshaw, and Hindley. However, Brontë also expresses a certain appreciation for the natural world; Linton and Cathy Linton's ideas of heaven both involve peaceful afternoons in the grass and among the trees. Likewise, Hareton is actually a very noble and gentle spirit, despite his outward lack of civilization and his description as a "rustic" (299).
