Observation of actions done by others evokes a motor resonant response, mediated by the mirror neuron system (MNS), which reflects the motor program encoding the observed action. Whether this response codes for the movements composing an action or for its motor intention remains controversial. Here, action and intention are dissociated by having subjects observe two different actions, a possible (palmar) or an impossible (dorsal) finger flexion, with the same grasping intention. Motor-evoked potentials (MEPs), elicited by single transcranial magnetic stimulation of the hand area in the left primary motor cortex of 12 subjects, were used to measure the excitability modulation of motor pathways during observation of the two different actions performed by an avatar's hand. MEPs were recorded simultaneously from the right Opponens Pollicis (OP) and Abductor Digiti Minimi (ADM). A significant modulation was found in the OP, during observation of the grasping phase of the possible action only (p<0.001), and in the ADM muscle during observation of the hand opening phase in both possible (p<0.01) and impossible (p<0.01) actions. We conclude that motor resonance in the primary motor cortex represents the movements composing an action and not its intention, since the resonant response evoked during the observation of the impossible grasping, programming for finger opening and not programming for thumb closing, would not be useful in encoding the grasping intention of that action.