Halliday (2008: 13):
Observed from close up, language appears in the guise of text, instances of spoken or written discourse that can be perceived by the senses — that can be heard or seen. Observed from a distance, language appears as a potential, an open ended network of possibilities with certain statistical properties and having certain kinds of interrelationship with its eco-social environment. The text is a process of selecting, and a product of selection, from within this overall potential.