Halliday & Matthiessen (1999: 82):
From a typological point of view, construing experience in terms of categories means locating them somewhere in [a] network of relations.
… the mental distinction between ‘emanating’ and ‘impinging’ is … the distinction between ‘middle’ and ‘effective’.
[With] decoding clauses (those where Token = Identified/Medium) the passive [ie receptive] is … rather rare. By contrast, in an encoding identifying clause, where the Token is Identifier/Agent and the Value is Identified/Medium, passive [ie receptive] is more or less as frequent as active [ie operative] … but only the active [ie operative] will accommodate a further agency …