Jean-Paul Sartre (1905–80) ranks among the leading thinkers of the twentieth century. His philosophical treatises developed and defended an original picture of human nature. He also wrote successful novels and plays that dramatized his important philosophical insights. While editing a leading popular journal of ideas, he addressed many contested political issues of his era with acumen and commitment. In addition, he published several literary biographies (and a partial autobiography) to demonstrate his approach to comprehending individuals in their historical contexts. His works stimulated responses from many of his most important contemporaries, for example, Merleau-Ponty, Levinas, de Beauvoir and Camus.

Being and Nothingness is subtitled An Essay on Phenomenological Ontology. Phenomenology is the systematic study of types of consciousness (or structures of human being) and their relationships to their objects. Sartre is an existential phenomenologist; he believes that lived experience can be described directly in a way that will yield important philosophical results. His approach is opposed to Edmund Husserl's transcendental phenomenology, the aim of which is to produce certain truths by withdrawing from the existential commitments of pre-reflective, lived experience. Husserl believed that all mental states emanate from an indubitable ego. Sartre argues that this ego appears only in the self-observational, reflective attitude Husserl presupposed and claims that no such ego exists in pre-reflective life. To the extent that people experience a continuant self, it derives from the defining gaze of others.