Halliday & Matthiessen (1999: 318):
We saw that in the experiential grammar of the English clause there was a complementarity of perspective between the transitive and the ergative: processes may be construed either as ‘one participant is doing something, which may or may not extend to another participant that is being done to’, or as ‘one participant is involved in something, which may or may not be brought about by another participant that is the agent of it’. Probably all languages display this transitive/ergative complementarity in their transitivity systems; but at the same time it appears at different depths and in different proportions.