Halliday & Matthiessen (1999: 213):
The key to the construal of experience is the perception of change; the grammar construes a quantum of change as a figure (typically one clause) and sorts out figures in the first instance into those of consciousness (sensing and saying), those of the material world (doing & happening) and those of logical relations (being & having). The central element of a figure is the process; ‘things’ are construed as entities participating in processes, having different rôles, of which one is ‘that participant in which the process is actualised’ … ; hence the grammatical nucleus of the clause is the configuration of Process with Medium.