Halliday & Matthiessen (1999: 548):
We have tried to make the point that the human condition is such that no singulary, determinate construction of experience would enable us to survive. We have to be able to see things in indeterminate ways: now this, now that, partly one thing, partly the other — the transitivity system is a paradigm example, and that lies at the core of the experiential component of grammar.