The infant's theory of self-propelled objects.

"Theory of mind" is treated as a modular component of human social behavior and an attempt is made to find the origins of this component in the perception of the infant. According to the theory I describe here, the infant assigns a high priority to changes in motion and divides the world into two kinds of objects on the basis of this criterion: those that are and those that are not self-propelled. How the infant perceives these two kinds of objects is described by four basic assumptions. First, when the state of motion of a nonself-propelled object is changed by another object, the infant's principal hard-wired perception is causality; when a self-propelled object changes its motion without assistance from another object the infant's principal hard-wired perception is intention. Second, if two self-propelled objects are related in a special way--a relation called the BDR sequence--the infant perceives not only intentional movement but also one object as having the goal of affecting the other object. Third, the BDR sequence has a more powerful consequence: the infant perceives that the affected object intends to reciprocate. Fourth, the infant expects that reciprocation will preserve valence (not form), where valence is formulated either as the preservation/denial of liberty, or as an aesthetic response.