Halliday & Matthiessen (1999: 133):
Whatever the mode of occurrence of any figure, it will always unfold in time. This temporal unfolding is construed as an inherent property of the process itself, realised grammatically as tense and aspect; it thus serves to validate the distinction between process and participant. Whereas on the one hand in its manifestation as a process, the figure unfolds in time, in its manifestation as participant, on the other hand, it persists through time — whether or not the participant undergoes any change of state. The limiting case is a creative or destructive process, such as writing or erasing a symbol, through which a participant comes into being or ceases to exist.