Born in 1905, Jean-Paul Sartre was one of the most influential philosophers of the twentieth century. Sartre was interested in the nature of existence, freedom, responsibility, consciousness, and time. Influenced by the work of Soren Kierkegaard, Friedrich Nietzsche, and Edmund Husserl, Sartre helped to develop a philosophical movement called existentialism. Although other contemporary philosophers, such as Albert Camus and Maurice Merleau-Pony, resented this classification, they all shared common ideas about experience, reality, and the preeminence of existence.
Sartre defined existentialism as the doctrine that "existence precedes essence." He distinguished between inanimate objects, or a "being-in-itself," and human consciousness, or a "being-for-itself." For example, consider a computer mouse. Its essence is the quality or qualities that one would use to describe it, such as its shape, color, smoothness, and weight. Its existence results from the fact that it plainly is. This distinction means that the observer "creates" the essence of the object simply by being conscious of it. The computer mouse is thus a "being-in-itself": its character has been assigned to it. But a person's emotions are not the same as a mouse's color. Sartre claimed that if one is happy it is by their own free choice. In this sense, humans exist and then define and choose their essence. Someone who has no fixed character consciously decides their essence and is thus a "being-for-itself."
Unlike traditional philosophical movements such as rationalism and humanism, existentialism rejects the use of scientific objectivity in understanding human existence. One such traditional philosopher, Wilhelm Friedrich Hegel, overshadowed the individual under a universal unity of truth. Instead, existentialists emphasize that the individual's consciousness, meaning the ability to be aware of facts, feelings, and sensations, is what determines reality. From this standpoint, existentialism addresses the limitations of human nature. Sartre believed that "reality" meant the objects he was conscious of at a particular moment in time, meaning that one could make complex decisions with only a limited amount of knowledge and in a limited timeframe.
Although many students turn to Sartre's most acclaimed text, Being and Nothingness, for an introduction to existentialism, the roots of many of Sartre's ideas originated in his 1938 novel, Nausea. Written just after the devastating Spanish Civil War and published right before the outbreak of World War II, Nausea addressed and anticipated the themes of anguish and despair that would come to define the horrors of the twentieth century. Sartre used the novel to expose the bare existence of objects and people. The novel's protagonist, Antoine Roquentin, is horrified to first confront the overwhelming existence of both objects and himself (instead of their essences) and then discover that there is to purpose to existence. Instead, he finds only nothingness: a vacuum filled by questions of free will, self-deception, and responsibility that still influence approaches to art and philosophy today.