Halliday & Matthiessen (1999: 213):
While participants are located in referential space, processes are located in time. The verbal group realising a process constructs a “moment” in time beginning with the ‘now’ (the time of speaking) leading up to the categorisation of the Event; this is analogous to the way the nominal group, realising a participant, constructs a “body” in space beginning with the ‘here’ and leading up to the categorisation of the Thing.