And in this diagram (after Halliday & Matthiessen 2004: 296 Figure 5.38) we can see the ergative functions of mental clauses represented
as a nucleus of Process + Medium/Senser,
with its additional participants of Agent/Phenomenon in impinging clauses,
and Range/Phenomenon in emanating clauses.
So, the question is: why should the theory of experience that naturally evolved in the English language come up with the property of bi–directionality in just these two process types, identifying and mental?
Well, the simplest explanation of a common property is a common origin, and if identifying relations precede sensing, then the simplest explanation — the one with the fewest additional assumptions — is that mental processes emerge from identifying processes. The diagram on the next slide suggests how that this might happen.