Thematically, Sula might be best summarized as a domestic novel: it's primary thematic concern is with the dynamics of family life. More specifically, Morrison focuses on the bonds of motherhood and sisterhood. Though these bonds give characters great strength and fill their lives with fulfilling companionship, they also lead to heartache and strife in some circumstances. The case of Sula Peace demonstrates the duality inherent in any close relationship: when a tight bond is formed supreme trust is given, but this trust is often betrayed.

Though Sula has real sisters, it is her adoptive sister, Nel Wright, that Morrison casts in the lead role for a drama about the complex bonds of sisterhood. Nel, sheltered by her aristocratic mother, takes to the free spirited Sula quite quickly: "Their friendship was as intense as it was sudden. They found relief in each other's personality." Sula is attracted to Nel in part because her life has a sense of order and formality her own boarding house existence lacks. Thus, Morrison sets up this close friendship as a classic example of the attraction between opposites. The friendship thus fostered gives the two young girls multiple opportunities for learning about the joys and burdens of womanhood: "Joined in mutual admiration they watched each day as though it were a movie arranged for their amusement. The new theme they were now discovering was men." Of course, among the discoveries they make about men is their power over them. Walking through the town streets on summer days, the catcalls of the men arouse their notice of their own bodies. Unfortunately, they are also faced with the violence perpetrated by men against women, and Sula teaches Nel a valuable lesson about how one might face this threat. After weeks of harassment from a group of Irish boys, Sula finally grows tired of hiding from their torments. In a show of her own strength of will "she slashed off only the tip of her finger. The four boys stared open mouthed.... 'If I can do that to myself, what you suppose I'll do to you?'" For Sula, strength in the face of torment is the key to adult life.

The strength of the friendship between Sula and Nel is repeatedly tested in the novel. The earliest instance comes when, playing a cruel game with one of the town's children, Sula hurls him into the river. He never resurfaces, and Nel is charged with keeping Sula's terrible secret. This she does for many years, showing the strength of her bond to her surrogate sister. After they reach maturity, the bond between Sula and Nel is strained again when Sula, having returned from an absence of decades, seduces Nel's husband. Though Nel never forgives her childhood friend for this callous act, she does, in Sula's final days, reconcile herself somewhat. She manages to minister to some of Sula's dying needs and finally comes to terms with her friends' transgressions. Thus, a friendship runs its course, with various trials alternately weathered and endured.

The pseudo-sisterhood enjoyed by Nel and Sula is no less complex than the bizarre version of motherhood Morrison presents with Eva Peace. Clearly, Eva is a deeply committed mother. Her children, however, have a difficult time recognizing the beneficence in her mother's actions. Her only son, Plum, returns from World War I withdrawn and addicted to heroin. Though she gives him a secure space in which to exorcize his demons, Plum never recovers from the violence of his experiences. Like the heroine of one of Morrison's other novels, Eva decides that her duty as a mother demands that she put Plum silently out of his misery rather than allow him to wallow on. So, one night after wrapping her boy in a final embrace, Eva lights him aflame. This action baffles her daughters, but for Eva the act is one of liberation. Morrison thus emphasizes the strength of her matriarchal character.

Sadly, however, this strength of character comes into conflict with Sula's own stubbornness. When she returns from college, Sula wants nothing from her mother but her house. Eva, therefore, is relegated to an old-folks' home. This shows Morrison's concern for the treatment of the elderly in a society filled with selfish children. Clearly, Morrison recognizes the devaluing of the experiences of elders as dangerous. Thematically, though, it shows the universal problem of unappreciative children. A mother's gifts to her offspring are, Morrison suggests, too frequently overlooked.

Though Morrison's consideration of the effects of racism is a social concern, her meditations on the causes of racial strife must be considered among her thematic interests. In addition to her novels, Morrison has written quite lucidly and astutely on the root causes of racism. Her collection of essays, Playing in the Dark: Whiteness and the Literary Imagination (1992), confirmed Morrison as a learned cultural critic and theorist. The reputation she acquired as a lecturer in various universities is confronted imaginatively in Sula. At numerous points in the novel Morrison considers how the white people of Medallion define, themselves against the black residents of the Bottom. At one point, she explains that for white outsiders ignored by Medallion's established residents, the black citizens become the only group over which to assert their superiority; in this, the town supports them: "As a matter of fact, baiting them was the one activity that the white Protestant residents concurred in. In part their place in this world was secured only when they echoed the old residents' attitude toward blacks." The Irish immigrants she describes here can only gain the respect of their Protestant neighbors by joining in their racist practices. Eventually, Morrison asserts, differences in national identity and religion become subordinated by an overriding difference: race.

Poverty and Hopelessness
Throughout the novel, the lives of the characters are shaped by poverty, as they have little or no money, unlike many of their white counterparts in the town. Although no one in the book is rich, the people of the Bottom are exceptionally poor. Eva has money only because she sacrificed her leg; others must make do as they can, with menial jobs or no jobs, because work for African Americans is limited by the racism of those who could hire them. When characters have dreams, like Jude, who dreams of doing a man's work on the road crew instead of spending a menial day as a waiter, they are crushed.

Existence in the Bottom is precarious at best, and is easily disrupted. Near the end of the book, people's hopes are raised by rumors that the new tunnel construction would use African-American laborers, and by the fact that an old people's home that was being renovated would be open to African Americans. However, these hopes are forgotten when a freezing rain kills all the late crops, kills chickens, splits jugs of cider, and makes the "thin houses and thinner clothes" of the Bottom people seem even thinner. Housebound, they make do with what they have, since deliveries have stopped and the good food is all being saved for white customers anyway. Thanksgiving that year is a meal of "tiny tough birds, heavy pork cakes, and pithy sweet potatoes." By spring all the children are sick and the adults are suffering from a variety of ailments.

All this suffering and malaise is accompanied by "a falling away, a dislocation." Mothers slap their children and resent the old people they have to take care of, wives and husbands become alienated from each other, and people begin bickering about small things. Christmas that year is a misery because of the sickness, lack of good food, and absence of money for gifts. The only gifts they can get are bags of rock candy and old clothes, given away by white people.

This feeling of doom and hopelessness leads almost everyone in town to participate in that year's celebration of National Suicide Day, with a feeling of reckless abandon at the idea of "looking at death in the sunshine and being unafraid," as well as the feeling of "this respite from anxiety, from dignity, from gravity, from the weight of the very adult pain that had undergirded them all those years before...as though there really was hope." This is the same hope that has kept them laboring in white men's beanfields in hopes of bettering themselves, fighting in other people's wars, kept them solicitous of white people's children, "kept them convinced that some magic 'government' was going to lift them up, out and away from that dirt, those beans, those wars." In other words, it's a futile and misguided hope.

Caught up in the energy of the moment, seeking release, the crowd of people pours on down the New River Road toward the tunnel, where they see "the place where their hope had lain since 1927. There was the promise: leaf-dead. The teeth unrepaired, the coal credit cut off, the chest pains unattended, the school shoes unbought, the rush-stuffed mattresses...the slurred remarks and the staggering childish malevolence of their employers." They try to destroy the tunnel, but in their desire to destroy it, they enter it and ultimately destroy themselves when the tunnel collapses under their attack.

Good and Evil
A major theme running through the book is good versus evil, and the fact that what people think is evil may be good, and vice versa. Shadrack, who appears in the first chapter, is considered dangerous and evil by the townspeople, and when he says "Always" to Sula, she takes it as a threat. However, he is not evil, he is simply shell-shocked and misunderstood; throughout the book, he never harms anyone. Sula is also considered evil, especially in the second half of the book, and Nel is considered good, but by the end of the book, Nel realizes that she has evil thoughts and has done evil things, while Sula has inspired the most good acts that the town has ever seen.

Eva, Sula's grandmother, is considered good, respectable, and a pillar of the community, but actually has a darker side. Her ruthlessness is hinted at by the rumor that she arranged to have own her leg cut off, a scene that is reflected by Sula when she cuts off the tip of her own finger to frighten off some harassing white boys. If she's able to do that to herself, she tells them, they should just think about what she'd be able to do to them. Sula's minor act of self-mutilation pales in comparison with Eva's, and the unspoken question the book asks is, "If she's able to do that to herself, what would she be willing to do to someone else?" The answer is, "Anything and everything," including killing her own son by pouring kerosene over him and setting him on fire while he's in a drug-induced haze.

Racism
The novel explores the relationship between the races, which is marred by racism and bigotry. In the opening scene, the founding of the Bottom is described; according to local legend, the area became the property of African Americans when a white man deceived a slave into thinking the high, dry, and eroded land was good for farming because it was the "bottom" of heaven. When Chicken Little is drowned, his body is found by a white man, who has no compassion for the dead child or his family, but who is merely annoyed at having to deal with the mess. On the train south, Helene and Nel experience degrading treatment at the hands of the white conductor and the white-run train system, which does not provide restrooms for African Americans. When Jude tries to get a job with the road-building crew, he is denied one, although the company hires scrawny whites who obviously can't do as good a job as he can; he can only get a job as a waiter, which he feels is servile and degrading. When Sula returns to town after a ten-year absence, her erratic behavior causes the townspeople to spread rumors about her causing all of their misfortunes, and the most damning rumor about her is that she willingly sleeps with white men.

Mothers and Daughters
Throughout the book, the many mother-daughter pairs have strained, unhappy relationships, and the lack of love a mother has for her daughter is passed on through the generations. In Nel's family, her grandmother, Cecile, disapproved of Rochelle, her prostitute daughter, and took Helene, Rochelle's daughter, away from Rochelle. Rochelle and Helene don't even know each other and are as alienated as Rochelle was from her mother. Nel, Helene's daughter, who is similarly alienated from Nel, feels oppressed by her mother's strictness and propriety, and feels stifled in her quiet, orderly house.

Eva, Hannah's mother, is an outwardly upstanding and secretly ruthless woman, and it's clear that her daughter, Hannah, didn't feel loved by her. At one point, she even asks Eva if she loved her children, a question that makes Eva angry. Hannah is also ambivalent about her daughter, Sula; Sula overhears her telling some friends that although she loves Sula, she doesn't like her, a comment that deeply wounds Sula. Because of this, Sula grows up feeling unloved and left out.