Halliday & Matthiessen (2014: 222-3):
The units that realise the process, participant, and circumstance elements of the clause make distinct contributions to the modelling of a quantum of change. The elements that make up the ‘centre’ of the clause – the process and the participants involved in it – construe complementary facets of the change. These two facets are transience and permanence. Transience means that a phenomenon is construed as unfolding through time by a verbal group serving as the process. Permanence means that a phenomenon is construed as continuous through time, being located in (concrete or abstract) space, by nominal groups serving as participants. Thus participants are construed as being relatively stable through time, and an instance of a participant can take part in many processes … In contrast, processes are ephemeral; every instance is a unique occurrence …
This contrast between participants and processes explains why there are names of individual participants — ‘proper names’, as well as names of classes of participants — ‘common nouns’, but only names of classes of processes: all lexical verbs are ‘common’ verbs.