Halliday & Matthiessen (1999: 222):
In addition to setting out the main dimensions and systems of ideational meaning, we have tried with this approach to bring out a small number of very general motifs that run throughout the grammar's construal of experience.The first of these motifs is that of meaning as expansion: the way regions of semantic space are opened up and defined by the three vectors of elaboration, extension and enhancement — elaborating a region that is already as it were staked out, extending the regions boundaries to take in more, and enhancing the region's potential by enrichment from its environment.The second motif is that of meaning by projection: the way new dimensions of semantic space are created by the orders of human consciousness, sensing and saying — by projecting into existence another order of reality, one that is constituted by language itself.