JOINT ACTION AND GROUP ACTION MADE PRECISE

The paper argues that there are two main kinds of joint action, direct joint bringing about (or performing) something (expressed in terms of a DO-operator) and jointly seeing to it that something is the case (e xpressed in terms of a Stir-operator). The former kind of joint action contains conjunctive, disjunctive and sequential action and its centr al subkinds. While joint seeing to it that something is the case is ar gued to be necessarily a intentional, direct joint performance can als o be nonintentional. Actions performed by social groups are analyzed i n terms of the notions of joint action (basically DO and Stit). A prec ise semantical analysis of the aforementioned kinds of joint action is given in terms of ''time-trees''. With each participant a tree is con nected, and the trees are joined defining joint possible worlds in ter ms of state-expressing nodes from the trees. Sentences containing DO a nd Stit are semantically evaluated with respect to such joint possible worlds. Intentional joint actions are characterized in terms of the n otion of ''we-intention'' (joint intention), characterized formally by means of a special operator.