Halliday & Matthiessen (1999: 181):
Both the difference in temporal permanence and the difference in experiential complexity are reflected logogenetically. Participants tend to persist in the unfolding of a text; and since they do, they can accrue various qualities. In contrast, processes cannot persist in text: unlike the deictic system of the nominal group, the deictic system of the verbal group, the tense system, is not a system for tracking textual instances of processes as a text unfolds. To achieve persistence in a text, processes have to be construed metaphorically as participants. When processes are construed as if they were participants, they can be established and maintained as referents in a text; hence under these conditions they also can accrue various qualities.