So long as the payments made to the state were irregular, it was, or at least seemed to be, for the interest of the crown to obtain from each province as much as it could anyhow pay.

When the state dues were once fixed, as the crown gained nothing by the rapacity of its officers, but rather lost, since the province became exhausted, it was interested in checking greed, and seeing that the provinces were administered by wise and good satraps.
The control of its great officers is always the main difficulty of a despotic government, when it is extended over a large space of territory and embraces many millions of men.

The system devised by Darius for checking and controlling his satraps was probably the best that has ever yet been brought into operation.

His plan was to establish in every province at least three officers holding their authority directly from the crown, and only responsible to it, who would therefore act as checks one upon another.

These were the satrap, the military commandant, and the secretary.