Halliday (2008: 163):
This [ideational metaphor] is a designed, or at least semi-designed, extension of the “experiential” way of looking at phenomena. It suits the “crystalline”, written mode of being; and in particular, as already said, it suits the elaborated discourses of organised knowledge, because it is good to think with — it enables you to build well-ordered conceptual structures and to spin tangled skeins of reasoning. High lexical density is the price to be paid.