Diagnostic: Clause Constituency
Halliday & Matthiessen (2004: 206):
Thus while ‘fact’ clauses serve as the Phenomenon of a ‘mental’ clause and can therefore be made Subject and be theme–predicated, ‘idea’ clauses are not part of the ‘mental’ clause but are rather combined with the ‘mental’ clause in a clause nexus of projection.
'Fact' clauses realise the Range of sensing, whereas
'idea' clauses realise the projection of sensing.