Étienne Souriau (1892-1979) was a renowned professor of aesthetics at the Sorbonne. In The Different Modes of Existence (1943), the artist's experience that dialogues with an unfinished work but that already exists by itself, becomes the key to a cosmology. Because everything, a work of art, a philosophical concept, a scientific object, but also a rock or a table, and ourselves, already exist but unfinished, halfway, as in twilight. All existence then demands an effort, demands a "process of instauration", and therefore requires an art. Souriau says that his book is offered as a "grammar of existence" for that "art of existing". The modes of existence are the elements with which the works-existences are made: phenomenal, reic, imaginary, possible and virtual elements, together with the passages of each other, and the events. And they do not unify into prefabricated wholes - there is no subject or object as a starting point - but in combinations to be made, which through echoes and mutual responses, produce "second-degree existences", the "over-existences". The edition is accompanied by Souriau's article "On the way of existence of the work to be done" (1956), and by an introduction by Isabelle Stengers and Bruno Latour that presents the author and his work, and offers some clues about its importance for the contemporary debate about philosophy and the epistemology of science.