Halliday & Matthiessen (1999: 268):
It is these characteristics of relators [the transphenomenality and fractality of the expansion relations they construe] that make them particularly liable to migrate: to be displaced metaphorically from their congruent status (as paratactic and hypotactic conjunctions) and to appear in other guises in other locations — as minor processes (in circumstantial elements), as processes, as qualities and as things. Thanks to this metaphoric instability, relators are able to play a central part in the re-construal of experience that is a feature of the discourse of the sciences — that makes these discourses possible, in fact, and hence provides the semiotic foundation for the construction of scientific knowledge.