Halliday & Matthiessen (1999: 453):
The third position accepts both the material process model and the relational process model without trying to interpret one in terms of the other. According to this model, Socrates is white is not primarily like Socrates ran but is more directly related to Socrates was a philosopher and, by an additional step, to Socrates was the teacher of Plato. In this model, nuclear processes are verbs and participants are nominals (substantival or adjectival).