Since the founding of psychology as a discipline, one central question has concerned the developmental origins of the mind. What is the form and content of our earliest understanding of the social and physical world? Behavioral research from the past 30 years has shown that by their first birthdays, infants interpret the actions of other agents in terms of what they see and want, and how physical obstacles constrain and impose cost on actions. Here I argue that these findings cannot be understood without granting that infants have an integrated understanding of people and objects. Against 3 existing hypotheses about early psychological understanding (sensorimotor, teleological, and modular hypotheses), I submit that infants have an intuitive theory of psychology that operates over a world model. In Marr computational-level terms, this understanding is a computational framework of probabilistic forward and inverse action planning in which agents’ actions and mental states are constrained by, and can cause changes in, external world states. This proposal remains to be fully tested, and raises broader questions about the development of the mind and brain; thus, the paper ends with future interdisciplinary work across the disciplines of developmental psychology, computational cognitive science, and cognitive neuroscience.