Halliday & Matthiessen (1999: 106):
Throughout the semantic construal of human experience, there is a differentiation between two orders of reality: between the everyday reality of our material existence on the one hand and on the other hand the second-order reality that is brought into existence only by the system of language. This is a contrast between semiotic phenomena, those of meanings and wordings, and the first-order phenomena that constitute our material environment.