A variety of disciplines and research areas have separately studied the notions of action, agents and agency, but no integrated and well-developed formal ontology for them is currently available. This paper is a first attempt at bridging this gap, focusing especially on the relationship between agency and action.

The departure point is STIT logic, the most expressive among the current logics of agency. Agency is the relationship between an agent and the states of affairs it brings about, without referring to how this is done, i.e., the actions performed. Since ontological investigations are best done in a first-order framework, making explicit at the language level the domain of quantification, we first propose a first-order theory that is proved equivalent to the propositional modal logic STIT. The domain and language of this theory is then extended to cover actions, obtaining the theory we call OntoSTIT+.