Our aim here is to show that an enigmatic text of Avicenna's Glosses, devoted to the problem of evil, takes on its full meaning in the light of the last chapter of the šifā' on De generatione et corruptione. We will see how Avicenna, deepening and developing a cosmological argument already present in the commentary of Alexander of Aphrodisias on the De generatione, ends up building most of his theodicy on the relative incommensurability of the different celestial periods. All the problem of divine Justice is thus reinterpreted in terms of opposition between discrete and continuous. Finally we will show the importance that this development of Avicenna had in Western Christendom in the 14th century, and the complex relationship which Leibniz has with it in his recently edited reflections on the Great Year.