The simple answer is Being and Nothingness is a full-length philosophy masterwork by France’s (possibly the world’s) greatest 20th-century philosopher, the “bible” of existential philosophy, in which Sartre examines the nature, the conditions, and the assumptions of “being” (the latest in a long line of philosophers who have tackled the ontological question, from Plato through Descartes through Buber and a dozen other philosophers. The title sets up the Manichean fallacy at the base of all pre-Sartreian inquiries: existence and nonexistence are not mutually exclusive states (be aware that any paraphrasing of this idea is inadequate; the book is an exhaustive, technical, very sophisticated argument). Sartre introduces the possibility that we (Mankind) are not a pre-designed invention or construction, but rather a set of possibilities, and we (Mankind) invent ourselves as we exist – Jesus, Mohammed, Hitler, Stalin, Kennedy, and the corner grocer all have contributed to what Man is. The comparison he makes to a paper-cutter is telling, and simple: A paper-cutter was invented, that is, someone saw a need, designed a solution, and assembled objects to fulfill that design. That, according to Sartre, is not the case with Man. Inauthentic and authentic existence, then is living according to some design, some set of rules, some measurements that don’t exist but have worked their way into Man’s assumptions, or living free from any pre-existing conditions. This is a difficult idea to grasp (part of Sartre’s caveat) because of the assumptions we have made ever since we became a culture.
Because Sartre’s tome is so complicated, so philosophically technical, and so difficult to apply to our everyday decisions, he wrote a slim layman’s answer to the often-asked question: “If we are convinced that existentialism is an authentic ontological view, how do we act according to its tenets? That is, how do we make everyday ‘moral’ or ‘logical’ decisions?” That book is called Existentialism and Human Emotion, and is where the student of modern philosophy should start.