Even though it will take some time for Roquentin to understand exactly what is happening to both him and the objects and people he sees, he has already begun to address the major themes of existentialism: anxiety, suffering, freedom, and self-deception. In writing his diary, Roquentin first intends to objectively study the changes he has observed, much like traditional philosophers, such as Hegel and Kant, would have used the scientific method to solve problems. Roquentin wants his diary to "sees," "classify," and "determine," just like a scientist would study a phenomenon. But this soon proves impossible as the very qualities, or "essences," of objects he seeks to describe no longer make any sense to him. From the rotting piece of paper on the street to the dirty stone he holds in his hands, Roquentin is unable to see, classify, or determine exactly what he is looking at. This leads him to question his free will, since he is unable to touch these objects; he is afraid of something but still not sure what it is.
Roquentin tries to pass off these odd sensations to his feelings of solitude. He worries that living alone has allowed him to hide the truth of his loneliness from himself. He fears that his diary will exaggerate events and later notes that he wrote that "nothing new" had happened to him with a "bad conscience." He soon admits that something causes him to lie to himself since, "I am not in the habit of telling myself what happens to me." Sartre termed this phenomenon "self-deception," which resulted from an individual's inability to cope with their absolute freedom. He believed that human choice is subjective since each individual is different and unconstrained by laws, cultures, and traditions. They are thus free to do whatever they want, but they consequently must accept full responsibility for their actions. Fear and anxiety in the face of this vast responsibility leads individuals to hide both their freedom and responsibility from themselves by lying to themselves. For example, Roquentin tells himself that he is "no longer free."
Roquentin's research on the Marquis de Rollebon is important for three reasons. First, it introduces the relationship between existence and time. Roquentin looks to the past to find a reason for his existence in the present. But so many details about Rollebon remain a mystery that he is confronted with the inaccessibility of the past as well. This second theme leads Roquentin to believe that objectivity is impossible--that nothing can be proved. In fact, he soon feels that his writings about Rollebon are more about himself than the marquis. Third, and most important, the presence of Rollebon introduces the existentialist theme of duality. Sartre used Descartes' method of posing the cogito, the individual's consciousness and the "other" part of himself or herself that observes that consciousness. For example, Roquentin tries to draw as many parallels between the marquis and himself that he can. Even when he looks at himself in a mirror, he thinks of Rollebon looking in a mirror and how they are both unattractive. This duality is reinforced by the similarities in their names as well as the almost analogous spelling of Roquentin and Rollebon's mistress, Roquelaire. Roquentin's dualistic personality allows him to observe himself, but to his horror he does not recognize his own face. The "nothingness" he perceives in his face recalls the anxiety he felt while looking at the glass of beer and the crumpled piece of paper--but exactly what this "nothingness" is still eludes him.