A comment from the section, "Function of the Mirror Neuron in the Monkey..." (p. 172)
RC state, "... although we are fully convinced ... that the mirror neuron mechanism is a mechanism of great evolutionary importance through which primates understand actions done by their conspecifics, we cannot claim that this is the only mechanism through which actions done by others may be understood." (p. 172).
The existence of another mechanism for action understanding is a serious problem for the mirror neuron theory of action understanding. Here's why:
To evaluate the importance of the mirror system for action understanding, we must understand what the mirror system adds beyond action understanding achieved by other means.
It's like discovering the existence of a 40 horsepower electric motor in a hybrid vehicle and claiming that the electric motor is the "basis" for the vehicle's power. This may be largely true if the gas-powered motor in said vehicle is a 5 horsepower Briggs & Stratton, but if it is a 700+ horsepower Formula One race engine, then "basis" is probably the wrong word.
So what do we know about this other mechanism for action understanding? Not much. In fact, as far as I know, there is no monkey data on this question. E.g., no one has tested whether monkeys fail to understand actions when the mirror system is lesioned -- RC further claim that such a study is not feasible for reasons we will highlight later. So this means that there is no basis for the claim that the mirror system plays a fundamental role in action understanding in the monkey, because the claim has never actually been evaluated empirically. In humans, the evidence in connection with speech suggests that the mirror system (assuming it exists in humans) plays a secondary role at best in action understanding: damage to frontal motor speech systems can destroy speech production without dramatically affecting speech understanding. Apparently, the other action understanding system is a pretty powerful system.
To summarize: RC's (correct!) admission that the mirror system isn't the only system for action understanding means that the claim that the mirror system is "fundamental" to, or at the "basis" of action understanding has never been scientifically tested in the monkey, and where it has been tested (human speech) has been shown to be incorrect.
So this is how the picture seems to be shaping up...
That other action understanding system:
Well, maybe it's not THAT lopsided. But how would we know? It's never been tested.