In the prologue of this book, Diodorus makes several statements that have been considered important for understanding the philosophy behind his entire work. Firstly, he announces the importance of parrhesia (free speech) for the overall moral goal of his work, insofar as he expects his frank praise of good people and criticism of bad ones will encourage his readers to behave morally. Secondly, he declares that the fall of the Spartan empire, which is described in this book, was caused by their cruel treatment of their subjects.