Halliday & Matthiessen (1999: 75):
… in his fourth year [the child] begins to extend his deployment of figures of being to include not only the ascriptive mode (‘a is a member of x’) but also the identifying mode (‘y equals x’) so that he can explicitly relate meanings in his ideation base in the form of definitions. […] He is now in a position to construe new meanings in terms of the semantic system itself; that is, they do not necessarily have to be ‘imported’ from direct extra-linguistic experience.