Roquentin's final refusal to research Rollebon gives him a greater understanding of the meaning of existence. He thinks the past does not exist at all, while the present is the only thing that does. His foray into Rollebon's past was nothing but a "vacation" from existence. Roquentin's memories of his research have become an empty vacuum just as Rollebon has returned to the "nothingness" from where he came. He realizes that anything he wrote about Rollebon and the past was pure fiction and better suited for a novel.
Roquentin's newfound appreciation and awareness of his existence puts him at odds with the people he meets and sees. While sitting at a café, he is disgusted with the robot-like behavior of the people around him. He thinks that they are trying to hide the enormous absurdity of their existence from themselves: they are either lying to themselves about whom they are going to sleep with or stuffing their mouths with food. Roquentin however, righteously declares that, unlike everyone else at the cafe, he does not hide his existence from himself: knows that he exists. His disgust causes him to get into a heated debate with the Self-Taught Man about humanism. The Self-Taught Man believes that all rational behavior can be explained by man's love for his fellow man. He proudly declares that he is a socialist and naturally loves every man and woman in the world. But Roquentin ridicules him for loving symbols and labels that are just essences and thus do not really exist.
The two men part on poor terms as Roquentin is overcome by the Nausea. He is afraid to touch anything for fear that it will make him sick. Holding a knife, he is shocked by the raw sensation of its handle. Suddenly, he understands what the Nausea is about: his fear of existence. Anything he touches no longer has any essence; it just exists. His revelation climaxes when he sees the root of a chestnut tree under a park bench. Roquentin can find no words to describe the root--it is simply there. He discovers that existence usually hides itself from view or thought with the facade of its essence, or attributes. For Roquentin, the word "existence" no longer evokes an abstract category but an unexplainable nothingness that has no reason to be there.
The second he tries to explain its size, color, or function, Roquentin discovers that he is no longer talking about the root, but about things that don't exist. He thinks back on the bartender's purple suspenders and realizes that they were never purple, but something that looked like a "color." He concludes that an object's essence is a simplified idea to hide its existence. The uneasy feeling of his Nausea is a result of colors, tastes, and smells that are not real. His feelings of Nausea also come from what he calls "contingency." He thinks that people attribute essences to objects to supply a reason for their existence. But he claims that there is no necessary reason for something to exist--it is there by accident. Since human existence is contingent, meaning anything can happen at any moment, Roquentin finds no reason for existence: it is just a free gift.
Roquentin finally understands what has been bothering him: the meaning of his existence. He first realizes that most people do not confront their own existence but tend to shy away from it. Roquentin himself had been using the Marquis de Rollebon to hide from his own life in the present: he had tried to deny his own existence by living second-hand through Rollebon. When he decides that the past is a meaningless vacuum, he must also accept the reality of his existence in the present. But when he does this, Roquentin is first shocked to find nothing and then disappointed to find that this "nothingness," is the meaning he has been looking for.
Despite his disappointment, Roquentin embraces the idea of existence. He endlessly repeats the phrase, "I exist," and declares that he could not stop existing even if he wanted to. Roquentin feels "free" because of his realization and criticizes the self-deception of other people who fail to acknowledge their own existence. His discovery of the "nothingness" behind existence leads Roquentin to oppose the Self-Taught Man's reliance on humanism. He does not believe that reality is the result of human rationality and love. Roquentin is not heartless but insists instead that humans must acknowledge the "nothingness" which makes man an accidental and unimportant aspect of reality. This proof ironically comes from Charles Darwin's "rational" theory of natural selection: human beings are not the center of the world but a lucky offshoot of different species. Sartre rejects the traditional philosophical study of groups or crowds of people, insisting that each individual must face up to the "nothingness" of reality.