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 a 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 a categorisation of the Thing. But while the Thing is enmeshed in an elaborate taxonomy of things, the Event is taxonomically rather simple and its complexity lies in the construal of time itself. Hence the verbal group is lexically sparse — typically the Event is the only lexicalised part; whereas nominal groups can be lexically extremely dense.