 Agency is sometimes exercised in concert, as when we walk together, several individuals undertake painting a house, or a football team executes a pass  play.[1]  It is hardly controversial that there really is a phenomenon falling under labels such as shared activity, and joint or collective action. What is disputed is how to understand it. One way to approach the issues here is to ask what distinguishes actions of individuals that together constitute shared activity from those that amount to a mere aggregation of individual acts. What is left over when we subtract what each of us did from what we did  together?[2]