Halliday & Matthiessen (2014: 29-30):
As grammarians we have to be able to shift our perspective, observing now from the system standpoint and now from that of the text; and we have to be aware at which point we are standing at any time. … Writing a description of a grammar entails constant shunting between the perspective of the system and the perspective of the instance.
Blogger Comment:
Note that speaking doesn't involve shunting up and down the cline of instantiation. Martin's notion of "distantiation" or "de-instantiaton" arises from this misunderstanding.