Halliday (2008: 45):
Survival depends on both aspects of our existence. Language has to be both a way of thinking, of construing the world, and a way of doing, of acting on and interacting with the people in it. This is in fact another complementarity; language acts out this complementarity, at a more abstract level, by transforming both thinking and doing into meaning. It is this that has shaped the evolution of lexicogrammar.