How do I know that I am the person who is moving? According to Wittgenstein (1958), the sense of agency involves a primitive notion of the self used as subject, which does not rely on any prior perceptual identification and which is immune to error through misidentification. However, the neuroscience of action and the neuropsychology of schizophrenia show the existence of specific cognitive processes underlying the sense of agency--the "Who" system--which is disrupted in delusions of control. Yet, we have to be careful in the interpretation of such clinical symptoms, which cannot be so easily reduced to deficit of action monitoring or to lack of action awareness. Moreover, we should refine the definition of the sense of agency by distinguishing the sense of initiation and the sense of one's own movements. A conceptual analysis of the empirical data will lead us to establish the taxonomy of the different levels of action representations.