Halliday & Matthiessen (2014: 333):
It is true that, from one point of view, all these types of process are different. Material, behavioural, mental, verbal, relational and existential processes each have a grammar of their own. At the same time, looked at from another point of view they are all alike. At another level of interpretation, they all have the same grammar: there is just one generalised representational structure common to every English clause.
These two perspectives complement one another, giving us a balance in the account of transitivity between similarity and difference among the process types. The two perspectives constitute two different modes of modelling transitivity. We shall call these the transitive model and the ergative model of transitivity.