Halliday (2008: 141):
When we are observing and investigating language, or any other semiotic system, our vision is essentially trinocular. We observe the phenomenon we want to explore — say, the lexicogrammar of language — from three points of vantage. We observe it from above, in terms of its function in various contexts. We observe it from below, in terms of its various modes of expression. And thirdly, we observe it from its own level: from within, or from round about, according to whether we are focussing on the whole or some of its parts.