From arbitrary sets to social groups Group agency in multi-agent modal logic

We differentiate between arbitrary sets of agents and organised groups. The difference is especially consequential in our strategic and moral reasoning. We expect group members to act towards their common interest, and we tend to hold individuals morally (and even legally) accountable for the joint actions of the groups they are members of. The distinction between groups and arbitrary sets of agents is mostly neglected by modal logicians who formalise reasoning about multi-agent systems. In logics of several group notions -- common knowledge, coalitional powers in games, and collective seeing-to-it-that -- groups are idealised as nothing but sets of agents. Such an idealisation is conceptually questionable, and it adds unnecessary technical complexity to the formalisms. This thesis is dedicated to filling this lacuna by introducing a philosophically informed definition of group agency into multi-agent modal logics.