Halliday & Matthiessen (2014: 223-4):
Change is thus construed as involving both transience and permanence, and the phenomena of experience are construed either as transient processes or as permanent participants. The border between these two is indeterminate; the lexicogrammar of every language will allow considerable discretion in how phenomena are treated in discourse, and lexicogrammars of different languages draw the borderline in different places. … This is an area of considerable fluidity; but most phenomena are treated as either as process or participant, and have to be reconstrued metaphorically to change their status in the grammar …