Halliday & Matthiessen (1999: 11):
The ideational semantic resources construe our experience of the world that is around us and inside us. The phenomena of our experience are construed as units of meaning that can be ranked into hierarchies and organised into networks of semantic types. The units of meaning are structured as configurations of functions (rôles) at different ranks in the hierarchy. For instance, figures are configurations consisting of elements — a process, participants and circumstances; these figures are differentiated into a small number of general types — figures of doing & happening, of sensing, of saying, and of being & having.