Halliday & Matthiessen (1999: 569):
In addition to the different degrees of their awareness of different grammatical units, such as words and clauses, people are also not equally aware of the different kinds of functions in which the resources of language are organised. In particular, in constructing and reasoning about more conscious models, people are readily aware of those linguistic resources whose function it is to interpret and represent experience, those of the ideational metafunction; but they are much less aware of those of the other two metafunctions, the interpersonal and the textual — no doubt because these do not embody representations of experience but reflect our engagement with the world in different ways. However, although they tend to be overlooked when one comes to build a 'scientific' model of language and the mind, these other metafunctions are no less important than the ideational.