Whenever in a language there are two classes of noun phrases such that members of one class are case-marked ergatively and members of the other class are case-marked accusatively, and there is a semantic difference between the classes related either to activeness of noun phrase referents, or their quantitative properties, or their pragmatic prominence, members of the class that ranks higher on these properties will be marked accusatively and members of the lower-ranking class, ergatively.
Standardized
Whenever in a language there are two classes of noun phrases such that members of one class are case-marked ergatively and members of the other class are case-marked accusatively, and there is a semantic difference between the classes related either to activeness of noun phrase referents, or their quantitative properties, or their pragmatic prominence, members of the class that ranks higher on these properties will be marked accusatively and members of the lower-ranking class, ergatively.
See also similar statements by Kozinsky 1980 (##472-474) and Silverstein 1976 (#217).