Chez les alchimistes greco-alexandrins, la fabrication reussie de la pierre philosophale depend des ingredients necessaires et des circonstances favorables pour l΄΄accomplissement du grand ouvre. C΄΄est ainsi que tous les travaux doivent avoir lieu aux temps prescrits et leur duree ne pas exceder certaines limites temporelles, la pierre philosophale etant consideree comme un etre vivant et sa fabrication comparee a la naissance d΄΄un etre. In the alchemical tradition in Greco-Alexandrian Egypt, the making of the philosophers΄΄ stone depends both on the ingredients used and the favorable conditions for the accomplishment of the great work. Consequently, the alchemists should work all phases in a specific time, and the length of time should not exceed certain limits because they considered that the philosophers΄΄ stone was a living being, as they compared its making to the birth of a human being.