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 text, processes have to be reconstrued 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.