As a Piagetian, Avery would consider using symbols the highest accomplishment of the sensorimotor period of development.
Piaget developed the theory of various stages of cognitive development. The first stage of cognitive development is the sensorimotor stage (0-2 years of age). The sensorimotor stage of development in turn consists of six sub stages. The most advanced and highest accomplishment of the sensorimotor period of development is when children are able to use symbolic thought to form mental representations of objects. In other words, children at this advanced sub stage are able to mentally visualize objects that are not physically in their field of vision.