Halliday & Matthiessen (1999: 559-60):
In those figures where there is a second direct participant, some form of agency runs through all the different types of process; but agency is such a complex aspect of human experience that the grammar does not delineate it by a single stroke, but construes it by means of a fundamental complementarity, that between the transitive and the ergative perspectives. Thus figures involving two direct participants, such as Actor + Goal in the material, are aligned along two different axes: the transitive one, based on the potential extension of force (mechanical energy) from a doer to another entity; and the ergative one, based on the potential introduction of agency (causal energy) from another entity as external source. Thus the earthquake shook the house is construed both as 'earthquake + shake' plus optional Goal 'house', and as 'house + shake' plus optional Agent 'earthquake'. As always in cases of complementarity, certain parts of the region are more strongly aligned to one perspective, other parts to the other, but the total picture requires the confrontation of the two.