The proper domain of naive psychological reasoning is human action and human mental states but such reasoning is frequently applied to non-human phenomena as well. The studies reported in this paper test the validity of the currently widespread belief that this tendency is rooted in the fact that naive psychological reasoning is initially restricted to, and triggered by, the perception of self-initiated movement of agents. We report three habituation experiments which examine the necessary conditions under which infants invoke a psychological principle, namely the principle of rational action, to interpret behaviour as goal directed action. Experiment 1 revealed that the principle of rational action already operates at 9 (but not yet at 6) months of age. Experiment 2 demonstrated that perceptual cues indicating agency, such as self-propulsion, are not necessary prerequisites for interpreting behaviour in terms of the principle of rational action. Experiment 3 confirmed that this effect cannot be attributed to generalisation of agentive properties from one object to another. These results suggest that the domain of naive psychology is initially defined only by the applicability of its core principles and its ontology is not restricted to (featurally identified) object kinds such as persons, animates, or agents. We argue that in its initial state naive psychological reasoning is not a cue-based but a principle-based theory.