This book sets out to define the precise meaning of Sartre's concept of 'being-in-the-world', following its introduction under the impetus of phenomenology, and to show the specific status of this concept in contrast to that established by its initiator, Heidegger. In doing so, he draws heavily on the work that Tony Ferri had already begun on Sartrean thought over twenty years ago, when he was studying philosophy at the Sorbonne in Paris IV. The challenge of the approach taken here is twofold. It lies both in highlighting the originality of Sartre's approach to what we might call the question of the world, and, correlatively, in understanding the roots and driving forces of human freedom, as reflected in the major characteristics of the figurehead of existentialism. At a time when naturalisms (racism, eugenics, neo-Darwinism, etc.), fatalism and intolerance are making a comeback, the interest of this book lies in the philosophical reinvestment of one of the greatest philosophies of freedom that our contemporary era has produced.