Halliday & Matthiessen (1999: 140-1):
A process of sensing may range over or be caused by a metaphenomenon, i.e. by a pre-projected fact serving as Phenomenon, as in (the fact) that she is late worries me. The two types of sensing that can involve a Phenomenon of this metaphenomenal type are the ones that cannot project, namely perception and emotion. That is, while perception and emotion cannot create ideas, they can 'react to' facts. In this respect, they are like certain relational clauses such as (the fact) that she is late is a worry/worrying — cf. (8) [agnate ascriptive process] below.