The novel by Sylvia Plath cannot be limited to only one central theme. Feminism, social pressure, gender inequality, sanity and mental diseases, mother-and-daughter relationships, body vs. mind, and personal ambitions are some of the controversial issues raised in the book. Still, the themes of gender inequality, depression, and body vs. mentality got the most comprehensive coverage in the book.
We cannot say that feminism is the central theme of The Bell Jar because the protagonist did not do anything to change the patriarchal state of things. But the depression (to be discussed below) resulted from flagrant gender inequality in all aspects of social life. Women did not have the same access to job openings and careers as men. They were criticized for having sex before marriage, and even more so if they had children as a result. Men did not face the same criticism. These and many other double standards afflict the protagonist and cause her anxiety.
Depression or mental illness vs. sanity is a theme that the reader follows throughout the novel. The symptoms accumulate and bring the main character to suicidal thoughts and actions. As the story shows us, depressive patients voluntarily isolate themselves, further worsening their condition. Thus, the best thing their friends and relatives can do is to initiate sincere conversations.
The lack of harmony between body and mind is another critical theme. Esther has mental problems, and her body suffers because of that. She wants to kill herself by destroying her body while she needs medical treatment for her mind. They are inseparable and equally require attention.