Halliday & Matthiessen (2014: 213):
Our most powerful impression of experience is that it consists of a flow of events, or ‘goings–on’. This flow of events is chunked into quanta of change by the grammar of the clause: each quantum of change is modelled as a figure — a figure of happening, doing, sensing, saying, being or having. … All such figures are sorted out in the grammar of the clause. … The grammatical system by which this is achieved is that of transitivity. The system of transitivity provides the lexicogrammatical resources for construing a quantum of change in the flow of events as a figure – as a configuration of elements centred on a process. Processes are construed into a manageable set of process types. Each process type constitutes a distinct model or schema for construing a particular domain of experience as a figure of a particular kind …