Halliday & Matthiessen (1999: 203):
Things of this category are the names of types of projection; in the taxonomy of things, they are semiotic abstractions, some discrete and some non-discrete, and as a grammatical class they are referred to under the heading of fact nouns. They are grammatically distinct because they can function as Thing in a nominal group with a fact clause as Qualifier, e.g. the notion that pigs can fly. Such clauses have the property that they can stand alone as participants in a figure, e.g. that pigs can fly is an intriguing notion.