Halliday & Matthiessen (1999: 25):
Thus an interpretation of semantics must account not merely for the system at some particular point in its evolution but also for the processes by which it got there and the changes that will shape it in the future. As far as text is concerned, the changes in semantic styling that take place in the course of a text cannot be dismissed as simply ad hoc devices for making the text shorter (or longer!), more interesting or whatever, they should be seen as the operation of general semogenic principles in the specific context which is engendering and being engendered by that text.