Halliday & Matthiessen (1999: 564):
From an educational point of view, the most fundamental complementarity is the move from the registers of everyday life to the registers of education: this is a move from folk or commonsense models to the "uncommonsense" models of systematic and technical knowledge. To say that the ideational system is polysystemic means that it can support these different theoretical angles on experience: semantic variation of all kinds is the manifestation of the different theoretical interpretations that language places on experience.