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 roles, of which one is 'that participant in which the process is actualised' (if there is 'flying', there has to be something that flies or is flown: birds fly, people fly kites); hence the grammatical nucleus of the clause is the configuration of Process with Medium.