What relation is there between the existence of a work of art and that of a living being? Between the existence of an atom and that of a value like solidarity? These questions become our own each time a reality--whether it is a piece of music, someone we love, or a fictional character--is established and begins to take on an importance in our lives. Like William James or Gilles Deleuze, Souriau methodically defends the thesis of an existential pluralism. There are indeed different manners of existing and even different degrees or intensities of existence: from pure phenomena to...