This paper introduces an action language for modelling the causality between a human’s motivational beliefs and behavior in human activities. The action language is based on a psychological theory, the theory of planned behavior (TPB), which centers on three sets of beliefs that shape an individual’s behavioral intentions: attitude, subjective norms, and perceived behavioral control. The language is modelled in the structure of a transition system in which states correspond to different mental states of an individual, and actions correspond to ways to transition between mental states in order to influence human intentions. We introduce its syntax and semantics. The semantics is characterized in terms of answer sets.