The paper is about the logic of expressions of the form 'agent x brings it about that A is the case', or 'agent x is responsible for its being the case that A', or 'the actions of agent x are the cause of its being the case that A'. Agents could be deliberative (human or computer) agents, purely reactive agents, or simple computational devices. The 'brings it about' modalities are intended to express unintentional, perhaps even accidental, consequences of an agent's actions, as well as possibly

more »

... intentional (intended) ones. Since we make no assumptions at all about the reasoning or perceptual capabilities of the agents we refer to this form of agency as 'unwitting'; unwitting can mean both inadvertent and unaware. The semantical framework is a form of labelled transition system extended with an extra component that picks out the actions of a particular agent in a transition, or its 'strand' as we call it. We de ne a modal language for talking about the actions of individual agents or groups of agents in transitions, including two de ned modalities of the (unwitting) 'brings it about' kind. The novel feature is the switch of attention from talking about an agent's bringing it about that a certain state of a airs exists to talking about an agent's bringing it about that a transition has a certain property. The middle part of the paper presents axiomatisations of the logic, and comments on relationships to other work, in particular on resemblances to P orn's (1977) logic of 'brings it about'. The last part is concerned with characterisations of (unwitting) collective agency, that is, the logic of expressions of the form 'the set G of agents, collectively though perhaps unwittingly, brings it about that A'.