It is very rare that I post on topics of which I actually have some knowledge, but I saw this and decided to make an exception. Mirror neurons:
...fire both when an animal acts and when the animal observes the same action performed by another (especially conspecific) animal. Thus, the neuron "mirrors" the behavior of another animal, as though the observer were itself acting.
Within the theory of mind, mirror neurons have often thought to the key to understanding the intentions of other people. For example, Peter Thier of Tübingen University:
...suggests this proximity-specific activity in mirror neurons may play an important role when we monitor what goes on around us, or serve as the basis for inferring the intentions of others and for cooperative behavior. 'These neurons might encode actions of others that the observers might directly influence, or with which he or she can interact,' he says.
We see someone waving their arm, for example, and we understand their act as as being the act of greeting BECAUSE the same act is recreated within us, as it were, by the firing of mirror neurons, and we know what the sensations mean in our own case.
Except, as been pointed out here, among other places, what mirror neurons mirror are basic physical sensations, the "feeling" of waving, or of lifting a cup of coffee to drink from it. But these specific physical sensations do not infallibly, or maybe even usually, indicate the presence of a particular intention on the part of the agent. I may use the same physical sensation to either shake your hand or poke you in the belly, so what the mirror neurons tell us CANNOT determine which of these things I am intending to do.
The only way to can get around this problem is by arguing that the mirror neurons reproduce not only the physical sensation, but recognize the intentions behind it and replicate these as well. But this is not a particularly satisfactory line of reasoning, because it explains how we recognize the intentions of others by saying that our mirror neurons do, which raises the question: how do they recognize these intentions?