Halliday & Matthiessen (1999: 243):
What is the status of such fictitious objects or pseudo-things? Unlike the other elements, which lose their original status in being transcategorised (for example shaker is no longer a process, even though it derives from shake), these elements do not: shakiness is still a quality, development is still a process — only they have been construed into things. They are thus a fusion, or ‘junction’, of two semantic elemental categories: shakiness is a ‘quality thing’, development is a ‘process thing’. All such junctional elements involve grammatical metaphor.