Halliday & Matthiessen (1999: 573):
As we noted above, folk models are part of the unconscious background of thinking in everyday situation types; they have evolved without any conscious design and are not associated with academic contexts. Folk models can also be more conscious, of course — these are the models that people talk about, that they believe they believe. Scientific models are consciously designed in more restricted situation types, usually within academic institutions, to serve as resources in reasoning about the world.