Independence
Independence is a theme that takes on different nuances at different points in the novel. When Roxana is abandoned by her husband the Brewer, independence is very stressful and traumatic for her; she does not have the money or the means to support herself and her children. She at first seems unable to function as an independent woman because of societal limitations, so she therefore has to seek out a relationship with the Landlord and become dependent on a man. However, once Roxana acquires enough money, she begins to be very attached to her independence: she becomes almost obsessed with obtaining financial security, and becomes mistrustful of the idea of marriage because she thinks that it will restrict her freedom and autonomy. Roxana's pursuit of independence makes her extremely wealthy, but it also eventually makes her lonely. She comes to feel that maintaining her independence is not as important to her as having a loving partnership with the Merchant, and possibly even building a relationship with her children. Also, virtually everything that happens in Roxana's life is reliant on her close relationship with Amy, showing that she is never totally independent and self-reliant. Rather, Roxana seeks a form of independence wherein she can choose whom she trusts and brings in to her life.
Moral Ambiguity
Throughout the novel, Amy and Roxana often debate and discuss the morality of certain actions, and whether or not it is acceptable to carry them out. Most notably, Roxana and Amy debate whether or not Roxana should have a sexual relationship with the Landlord/Jeweller, even though they are not married, and whether or not Amy should murder Susan. Roxana also has some debates with the Merchant about the benefits of marriage, and whether or not it serves women's interests. These debates highlight the difference in personality and class background between Amy and Roxana, as well as their deeper similarities. Amy is pragmatic, blunt, and street-smart; because she comes from a less privileged background, she does not have the luxury of being idealistic. Roxana is more tied to conventions of morality, and also worried about her reputation and social position. However, even though Roxana is also hesitant about some of these actions initially, she can see the value of them, and the opportunities they might create. In addition to highlighting this character development, the theme of moral ambiguity and debate allows Defoe to embed examples of casuistry (a kind of moral reasoning for resolving ethical problems) in to his novel.
Wealth
Wealth is a predominant theme in the novel because it essentially becomes Roxana's obsession. She learns early in life that wealth is perilous and can be lost through financial mismanagement; the trauma of going bankrupt and having to give up her children leads her to become obsessed with achieving financial security for herself. However, the novel hints that a focus on money can become a type of addiction and lead to unhealthy patterns of greed; once Roxana has acquired enough money that she is comfortable, she does not stop trying to acquire money. Instead, she wants to push the boundaries and see how much she is able to acquire. Roxana keeps pushing herself to gain more and more wealth until she can no longer recall why this was so important to her. She also prioritizes wealth ahead of relationships, and this almost ends up costing her her relationship with the Merchant, whom she actually loves. The novel is ultimately ambiguous about the theme of wealth, showing how it can be both a force of empowerment, but also a source of corruption.
Motherhood
Despite Roxana giving birth to many children, motherhood does not always seem to be important to her. However, the later part of the novel reveals that motherhood is indeed a prominent theme of the novel. Although she doesn't seem to think much about them while she is abroad, Roxana is very eager to find her five children once she returns to England. She not only provides for them financially, but she changes her whole lifestyle in hopes that she might be able to build relationships with them. Whenever Roxana is physically in the same space as her daughters, she has a very strong reaction to being close to them. It is also made clear that she is grief-stricken when she first has to give up her children. However, Roxana can be ambivalent about some of her other children; she actively dislikes the son she has with the Merchant, and tries to avoid spending time with him. She also seems unmoved when one of her sons with the Prince dies in Italy. Also, while she clearly does have some love for her daughter Susan, she only wants to love the girl on her own terms. Roxana does not want to ever admit the truth to Susan unless she can protect her own interests and identity. Roxana also seems somewhat unmoved by Susan's strong emotions about wanting to be claimed by her mother.
Secrecy & Lies
Because Roxana reinvents herself a number of times, and conceals her previous identity each time, she ends up living within a web of secrets and lies. Amy is the only person who ends up with full access to Roxana's history. Roxana's secrecy begins when she misrepresents herself as the legitimate wife of the Jeweller upon his death, in order to claim money and assets. After that, she stacks lie atop of lie, adopting other fake personas when she returns to England and when she moves in with the Quaker and tries to leave her scandalous past behind. Roxana's secrecy and lies are such a dominant theme that readers never even learn her true name, other than a hint that she might have been called Susan. While Roxana is very skilled and competent in the art of secrecy, she ends up always living in fear, no matter how wealthy and powerful she becomes. As is shown when Susan starts investigating, everything Roxana has built could come tumbling down if her true history is revealed. The theme of secrecy shows how lonely and isolating living a lie can be: Roxana loves both the Quaker and the Merchant, but she can never be truly honest with either of them. Part of the reason why Roxana is so attached to Amy is that Amy is the one person who truly knows her, and therefore Roxana cannot bear to be parted from her, even after Amy almost certainly murders her daughter.
Travel & Mobility
Throughout the novel, Roxana moves from place to place quite often, living in multiple countries, and often going on extended trips. The theme of travel and mobility adds narrative interest to the story, showing that she is glamorous and cosmopolitan. When Roxana makes her debut as a courtesan in London, her connection to the East via her Turkish costume marks her out as special and desirable. Roxana's ability to move from place also demonstrates that she has been successful at achieving her freedom as a wealthy and independent woman of means: she can go where she wants at a time when this would not have been possible for many women. However, Roxana's mobility is also necessitated because of her lies and fake identities. If she had not built her fortune on a web of lies, she would not have had to move around so much, and could have chosen a more stable life if she had desired. Indeed, many of Roxana's moves come from her needing to run away and avoid detection, not because she is going to places she desires to be.
Social Class
Social class is an important theme in the novel, especially since Defoe was writing at a time when the relationship between class and wealth was changing in English society. Aristocratic individuals who held titles and landed estates were not necessarily as wealthy or financially secure as individuals who ran a business and could make money for themselves, even though anyone from the professional classes would typically have been considered of a lower social station. Roxana comes to function as a businesswoman, making money and then investing it to increase her profits. She also sees the Merchant as the most financially secure man she can be with, and wants her own sons to pursue a similar career path. Nonetheless, Roxana longs for the status that comes with a title, hence why she persuades the Merchant to buy them titles in both England and Holland. However, by showing characters purchasing titles rather than inheriting them through legitimate succession, Defoe makes the point that the idea of class and social mobility had changed forever by this point in history.