Bergson is rightly considered the philosopher of duration. Has this theory, however, been sufficiently elucidated? Is there not a domain, aside from life itself, to which the characteristics of duration can be meaningfully ascribed? Why will Bergson speak in 1932 of a "real" duration, in relation to the theses of 1907? "Duration and Simultaneity. With Reference to Einstein's Theory" (1922) is the only volume written by Bergson in the period separating "Creative Evolution" (1907) and "The Two Sources of Morality and Religion" (1932). "Duration and Simultaneity" represents a polemical, unique, mature and relatively neglected work, one that allows us however to respond to the above questions - provided that we read it as a work of philosophy. Reality, far from identifying itself with mobility or with the moving, without coinciding with the real and even taking place so to speak 'beyond' duration, reality makes itself within Bergsonian metaphysics. This book was awarded the 2020 Polydore de Paepe Prize of the Royal Academy of Science, Letters and Fine Arts of Belgium.