Halliday & Matthiessen (1999: 412-3):
It is clear, then, that the ideation base supports the text base: textual information can be stated as patterns over the [ideation] base — partitions, and moves between partitions. At the same time, it is also very clear that the way the ideation base is organised — the various configurations that have evolved — must have been shaped by textual pressure. So, for instance, given that a relationship could be construed either as ‘that boy’s hair is green’ or ‘that boy has green hair’, the preference in English for the construal where the whole is Carrier and the part is Attribute can be understood in textual terms: it means that the whole can serve as Theme.