Although we can identify the relevant components, and factorize them into internal and external components, the nature of reciprocal coupling makes it difficult to study the components as separate systems because they are continuously influencing and responding to one another. They are coordinating with one another to produce behavior. (Menary, 2010, p. 4).Now, why would reciprocal coupling per se make it difficulty to study the components (brain and body) as separate systems? Even by the EC line, isn't this how things have been done up to this point? There has to be more to the story. The thermostat and the furnace continuously influence and respond to one another, but they look to be separable systems (or at least separable sub-systems or components of a single system).
Menary, R. (2010). Introduction. The Extended Mind. Cambridge, MA: The MIT Press. (pp. 1-25)