Halliday (2008: 44-5):
But the principle is usually formulated in terms of just one aspect of this relationship, the ideational: the way our language functions to construe our own experience of the world. Yet the human condition is a social one, and our environment is not just ecological — it is eco-social; so our language functions also interpersonally. And here it is performing a more active rôle: it is not just construing but enacting our relationships with our fellow creatures. In terms of the classical model deriving from Plato, there is the “first–&–second person” component of language, that which is organised around the interaction of “you” and “me”, as well as the “third person” component of language, that which is organised around the construal of others, of “him, her, it, them”. These two metafunctional components are at the foundation of every human language, and both evolved under the same conditions. Our social interactions increased in complexity just as did our relations with all aspects of the environment. Neither component can be instantiated without the other.