  Individuals do not always act alone. They may also share intentions and act in concert. There has been growing interest in the philosophy of action about how shared intention and action should be understood. A central concern is whether the sharing of intentions should be given a reductive account in terms of individual agency (see Searle 1990 for an important early discussion of the issue). Michael Bratman [1992] offers an influential proposal in a reductive vein that makes use of his planning conception of intentions. A central condition in his account of shared cooperative activity is that each participant individually intends the activity and pursues it in accordance with plans and subplans that do not conflict with those of the other participants. But Margaret Gilbert [2000] has objected that reductive approaches overlook the mutual obligations between participants essential to shared activity: each participant is obligated to the others to do his or her share of the activity, and unilateral withdrawal constitutes a violation of this obligation. Gilbert argues that a satisfactory account of these mutual obligations requires that we give up reductive individualist accounts of shared activity and posit a primitive notion of joint commitment (see also Tuomela, 2003).