It is the unique business of agents to act. But what does it take for an agent to act, and what unique conditions render an agent capable of acting? A remark of Wittgenstein’s proves useful in crystallizing this question: “…when ‘I raise my arm’, my arm goes up. And the problem arises: what is left over if I subtract the fact that my arm goes up from the fact that I raise my arm? (Are the kinaesthetic sensations my willing?)”. Wittgenstein suggests here that there is something more to our actions than the mere observable physical occurrences concomitant to them. What must be added to the event of one’s limb moving that would amount to the action of moving one’s limb? To put the problem more generally, what is the action-making ingredient that sets the things that we do apart from the things that merely happen to or around us? These questions call for a theory of action-explanation. Such a theory can clarify how actions come about, and can illuminate what we mean when we say that an agent has acted. Explicit attention to this fundamental philosophical question is of foremost importance to designing principled approaches to assessing the ethical valence of the downstream outputs of brain-computer interfaces and determining how (if at all) they differ from usual forms of action.