Halliday & Matthiessen (1999: 565):
As a resource for making sense of our experience, the ideation base enables us to construe a range of different theories, commonsense as well as scientific. There is a cline between folk, or commonsense, theories and scientific, or uncommonsense, ones; and at any point along the cline alternative theories may be in competition. The ideation base, by dint of being polysystemic, accommodates variation along this cline, not only from folk to scientific but also across alternatives: it embodies both congruent and metaphorical construals of experience, and it provides elasticity within the overall construction space.