Halliday & Matthiessen (1999: 75):
In his fourth year, Stephen begins to provide causal evidence for categorisation, by adding an enhancing figure. The figure that is linked causally specifies attributes that are critical to the construal. For example: Mother refers to Bond airship as 'spaceship balloon' — Stephen: Not a spaceship — an airship — cause a spaceship has bits like this to stand it up; Stephen (pointing at page numbers): That's fifteen because it's got a five; that's fourteen because it's got a four.
These causal relations are internal (Halliday & Hasan, 1976), i.e. oriented towards the interpersonal act of communication itself ('I know it is an x, because it has feature y') rather than external ('it is an x, because it has feature y').
In other words, Stephen is attending to the act of construal itself — 'I construe it as x because of y'. At this stage in the development of Stephen's construal of experience, the construal has become something that is not only shared, but can also be explicitly negotiated and argued about.