Halliday & Matthiessen (2014: 261-2):
In ‘relational’ clauses, there are two parts to the ‘being’: something is said to ‘be’ something else. In other words, a relationship of being is set up between two separate entities. This means that in a ‘relational’ clause in English, there are always two inherent participants – two ‘be-ers’. In contrast, the general classes of ‘material’ and ‘mental’ clauses have only one inherent participant (the Actor and the Senser, respectively). Thus, while we can have a ‘material’ clause with one participant such as she was walking or she was walking into the room, we cannot have a ‘relational’ clause such as she was, with only one participant; we have to have two: she was in the room. Similarly, a ‘mental’ clause with one participant such as she rejoiced is possible; but the nearest ‘relational’ equivalent must have two participants – she was happy, not she was.
Blogger Comment:
Note that attributive clauses with qualitative Processes, such as this stinks, manifest only one participant, unless the Attribute is interpreted as conflated with the Process; see Halliday & Matthiessen (2014: 271).