If material processes are those of doing and mental processes those of sensing, the other main category, relational processes, are those of being; for example 'Sarah is wise', 'Tom is the leader'. The central meaning of clauses of this type is that something is. But every language accomodates, in its grammar, a number of distinct ways of being, expressed as a different types of relational process in the clause.
- Halliday (1985), Pag. 112