Halliday & Matthiessen (1999: 267):
But things are merely the end-point of the metaphoric scale… . Processes, though more constrained than things, still have more semantic potential than relators: they accommodate categories of time and phase, among others, and are construed in open lexical sets, whereas relators for closed systems. So there is pressure there too, to metaphorise conjunctions into verbs: then, so, because, before, therefore becoming follow, result, cause, anticipate, prove. (Circumstances are something of a special case because most of them already contain participants in minor, subsidiary processes — prepositional phrases in the grammar.) But it remains true that things are the most susceptible of being classified and organised into taxonomies; hence the primary motif of grammatical metaphor is that of construing a world in the form of things.