Halliday & Matthiessen (1999: 1):
In this book we are concerned with how human beings construe experience. This means, first and foremost not experience as an instantial product — the particulars of the world that is around us and inside our heads, the particular individuals, the events of last Friday, and so on — but experience as a resource, as a potential for understanding, representing and acting on reality. It is in terms of this potential that the particulars of daily life are interpreted: they make sense because they are instantiations of this potential.