Inferring and Explaining

147 what a baby looks at can tell you something about how it sees the world. Tis is because babies are like adults in some regards. If they see the same thing over and over again, they get bored and look away. If they see something new or unexpected, they look longer. Tus, analyzing looking time can tell what babies think of as being “the same thing,” and what they see as “new or unexpected.” 13 The above two-stage inference to the best explanation—differential gaze times being explained as boredom or surprise and then as “same” or “new”—is themethodological presup- position for a host of fascinating experiments in the study of infant cognitive development. Paul Bloom provides a nice summary of some of these results: 1. Cohesion: If a hand pulls at an object, babies expect the entire object to go with the hand; if it comes of in pieces, they are surprised, showing an expecta- tion that objects are cohesive . 2. Continuity: Imagine a stagewith two vertical barri- ers separated in space. A small object, like a box, goes behind the barrier on the lef, continues between the barriers, goes behind the barrier on the right, and comes out the other side. Adults see this is a single object, and so do babies. Now imagine that a box goes behind the barrier on the lef, there is a pause, and then the box emerges for the screen on the right, never appearing in the gap. Adults assume there are two boxes here, not one. Babies make the same assumption; they expect continuity . 14 Why do we fnd diferential gaze times for the hand pulling the object and it remaining whole, and the hand pulling the object and it coming of in pieces? Babies expect objects to be cohesive. Why the perception of a single box in the frst experimental scenariowith the box and the barri- ers but the perception of two boxes in the second scenario? Babies expect continuity. But where do these expectations come from? Bloom’s answer is a classic blend of nature and nurture. Tese results show that although babies enter the world with a foundational understanding of what objects are and how they act, it is incomplete, and this foundation grows. Some of the improvement might be due to maturation of the brain—like the rest of the body, the brain changes rapidly in the early years of life, and this might cause corresponding increases in knowledge. But some of the improvement is plainly due to experience.” 15 And fnally, what explains this foundational understanding of objects and how they act? Tis knowledge is clearly innate. Natural selection has hardwired infant brains to expect cohesion and continuity. It is easy to see the adaptive value for human infants having rudimentary under- standing not just of objects and “folk physics” but also of agency and social relationships. Certainly, shared understanding of folk physics, agency, and social relationships are the cornerstones of the sort of the practical explanatory skill that would have been of value inhunter-gather times. Sally and Ann Let me tell you two stories about Sally and Ann. Sally prizes her special marble. When she leaves, she always places it in a basket and carefully covers it with a sof blanket. Ann has been hid- ing and watching Sally’s little ritual. Afer Sally has gone home for lunch, Ann removes Sally’s exPlanatory VIrtue and truth