9-12 months: Infants begin to have an understanding of object permanence - at first they're able to retrieve an object that's hidden if they've already started to search for it, and Piaget argued that the initiation of retrieval helps the completion of this operation. Soon afterwards, they're able to search for hidden objects, but they still don't fully understand that an object should be in the place where it disappeared last. Piaget suggested that the A not B error occurs because infants don't yet understand that there's only one object that they see, but probably assume that the object and its location are somehow associated together. The following study demonstrates the error they make, known as the A not B error. A toy is repeatedly hidden in one location (Place A). In one trial, it's moved in the child's view, to another hiding place (Place B). The child will search in the first location (Place A)