Halliday & Matthiessen (1999: 75-6):
Also in his fourth year, Stephen 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.
For example, in Balance means [[you hold it on your fingers and it doesn't go]], he construes a single class of abstract participant, 'balance', as something defined in terms of a sequence of figures, 'you hold it on your fingers and it doesn't fall off: see Figure 2-11.
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. As Painter points out, this is yet another expansion of his semantic resources preparing him for educational learning in later life where experience is very often vicarious experience, constructed entirely in educational discourse.