Halliday & Matthiessen (1999: 293):
We have considered how metaphorical examples might be represented as semantic configurations related to congruent configurations, very likely with several intermediate steps. We now have to take one step further and explore how semantic types can be represented as metaphorically related to congruent types within the overall semantic system. Such a representation has to meet the kinds of ideational and textual demands that we have already considered in our representation of examples:(i) ideational: the representation has to show that metaphor constitutes an expansion of the semantic system. The expansion is an elaborating one, creating chains of token-value relations; and it increases the semantic potential for construing experience.(ii) textual: the representation has to make it possible to show how metaphorical and congruent variants are given different values in the text base — in the textual semantics.