Nella sua Biblioteca Storica, Libro II. 39.5 (non in inglese):

As for the customs of the Indians which are peculiar to them, a man may consider one which was drawn up by their ancient wise men to be the most worthy of admiration; for the law has ordained that under no circumstances shall anyone among them be a slave, but that all shall be free and respect the principle of equality in all persons. For those, they think, who have learned neither to domineer over others nor to subject themselves to others will enjoy a manner of life best suited to all circumstances; since it is silly to make laws on the basis of equality for all persons, and yet to establish inequalities in social intercourse.

Ringrazio il Prof. Giovanni Ugas, che cita l’ultima parte all’inizio del suo Shardana e Sardegna:

Coloro che hanno appreso a non dominare gli altri, né ad assoggettarsi ad altri, godono di una vita migliore, adatta a tutte le circostanze; infatti è sciocco formulare le leggi in base all’eguaglianza di tutti e poi stabilire l’ineguaglianza nei rapporti sociali

Fonte digitale: Bill Thayer (http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Diodorus_Siculus/2B*.html)