Halliday & Matthiessen (1999: 537):
"Congruent" is of course a contingent term. What it is saying is that, at the present moment in human history we can recognise forms of language which seem to represent a common coding of experience: this is the configuration that we referred to as "process + participant + circumstance" which is construed in grammars through some version of the trichotomy of verb, noun and the rest. If we relate this to English, it is the form of English that is learnt as a mother tongue, in which phenomena are interpreted clausally, in a kind of dynamic equilibrium of happenings and things.
The prototypical thing is a concrete object which can be related by similarity to certain other objects, such that taken together they form a class, like engines. The prototypical happening is a change in the environment that is perceptible to the senses, or a change in the senser's own consciousness. A process is a happening involving one or two such objects, or one object and a conscious being.
When children move from their own constructed protolanguage into the mother tongue, this provides a theory which they can use to give a plausible construction to their own individual experience.