Agency and the logic of ability

Abstract

Theories of ability based on the dynamic logic of programs often presuppose that the agent has complete control over its actions to the extent that execution of the action never fails. Similarly, logical theories of ‘seeing to it that’, Belnap and Perloff (1988) and ‘bringing it about', Segerberg (1989), model the result of an action without regard to the original intention of the agent, so these logics are not of direct use to formalizing the reasoning of a planning agent which must make a judgement about the likelihood of its action succeeding. In this paper, we propose an analysis of simple ability, i.e. considering only atomic actions, which is compatible with both the present and future directed sense of intention, whilst admitting the possibility of action failure. The basic idea is that an agent has the ability to do an action A to achieve some goal G if it normally brings about G when attempting to do so by doing A. We shall assume a primitive notion of ‘attempting’ or, as Bratman (1987) calls it, endeavouring, to perform an action to achieve some goal. Thus goal-directed behaviour is central to defining ability. In the latter part of the paper, we argue that this concept is also central to defining agency. We propose that agency is best understood as self-controlled goal-directed activity, where the notion of an action being under the control of an agent is intimately tied to the agent's ability to perform that action successfully under normal conditions.