Halliday & Matthiessen (2014: 713):
The ‘re-mapping’ is possible because semantic motifs such as cause are manifested repeatedly in the different environments of the grammar so that each environment is a possible domain of realisation for such a motif. These motifs are of the two primary types, expansion and projection. Ideational metaphor is based on patterns that exist already in the congruent mode of realisation; but it expands these patterns significantly, as can be seen when we analyse scientific, legal or administrative discourse – or, indeed, other kinds of discourse that the metaphorical mode has spread to in a systematic way.