Halliday & Matthiessen (2014: 272-3):
… ‘relational’ clauses may construe both ‘outer experience’ [material] and inner experience [mental]. So both these modes of experience are included within the domains of attribution of an ‘attributive’ clause; but these domains transcend the two modes. In particular, ‘inner experience’ is generalised to include not only subjective sensations but also attributes that are construed as objective properties of macrothings [acts] and metathings [facts] … The general contrast in domains of attribution is thus not that of material vs mental but rather ‘material’ vs ‘semiotic’. The attributes assigned to the carrier in an ‘attributive’ clause are either material ones or semiotic ones, and the ‘thing’ serving as carrier has to be of the same order as the attribute. Thus with true as Attribute, the Carrier has to be a metathing – represented by a fact clause … or by it, that, this referring to a fact …