Sartre's primary idea is that people, as humans, are "condemned to be free". This theory relies upon his position that there is no creator, and is illustrated using the example of the paper cutter. Sartre says that if one considered a paper cutter, one would assume that the creator would have had a plan for it: an essence. Sartre said that human beings have no essence before their existence because there is no Creator. Thus: "existence precedes essence". This forms the basis for his assertion that because one cannot explain one's own actions and behavior by referring to any specific human nature, they are necessarily fully responsible for those actions. "We are left alone, without excuse." "We can act without being determined by our past which is always separated from us."
Sartre maintained that the concepts of authenticity and individuality have to be earned but not learned. We need to experience "death consciousness" so as to wake up ourselves as to what is really important; the authentic in our lives which is life experience, not knowledge. Death draws the final point when we as beings cease to live for ourselves and permanently become objects that exist only for the outside world. In this way death emphasizes the burden of our free, individual existence.
As a junior lecturer at the Lycée du Havre in 1938, Sartre wrote the novel La Nausée (Nausea), which serves in some ways as a manifesto of existentialism and remains one of his most famous books. Taking a page from the German phenomenological movement, he believed that our ideas are the product of experiences of real-life situations, and that novels and plays can well describe such fundamental experiences, having equal value to discursive essays for the elaboration of philosophical theories such as existentialism. With such purpose, this novel concerns a dejected researcher (Roquentin) in a town similar to Le Havre who becomes starkly conscious of the fact that inanimate objects and situations remain absolutely indifferent to his existence. As such, they show themselves to be resistant to whatever significance human consciousness might perceive in them.
He also took inspiration from phenomenologist epistemology, explained by Franz Adler in this way: "Man chooses and makes himself by acting. Any action implies the judgment that he is right under the circumstances not only for the actor, but also for everybody else in similar circumstances."
This indifference of "things in themselves" (closely linked with the later notion of "being-in-itself" in his Being and Nothingness) has the effect of highlighting all the more the freedom Roquentin has to perceive and act in the world; everywhere he looks, he finds situations imbued with meanings which bear the stamp of his existence. Hence the "nausea" referred to in the title of the book; all that he encounters in his everyday life is suffused with a pervasive, even horrible, taste—specifically, his freedom. The book takes the term from Friedrich Nietzsche's Thus Spoke Zarathustra, where it is used in the context of the often nauseating quality of existence. No matter how much Roquentin longs for something else or something different, he cannot get away from this harrowing evidence of his engagement with the world.
The novel also acts as a terrifying realization of some of Immanuel Kant's fundamental ideas about freedom; Sartre uses the idea of the autonomy of the will (that morality is derived from our ability to choose in reality; the ability to choose being derived from human freedom; embodied in the famous saying "Condemned to be free") as a way to show the world's indifference to the individual. The freedom that Kant exposed is here a strong burden, for the freedom to act towards objects is ultimately useless, and the practical application of Kant's ideas proves to be bitterly rejected.
Also important is Sartre's analysis of psychological concepts, including his suggestion that consciousness exists as something other than itself, and that the conscious awareness of things is not limited to their knowledge: for Sartre intentionality applies to the emotions as well as to cognitions, to desires as well as to perceptions. "When an external object is perceived, consciousness is also conscious of itself, even if consciousness is not its own object: it is a non-positional consciousness of itself."