Research on human infants and young children has provided evidence for five systems of core knowledge: knowledge of objects and their motions; of agents and their goal-directed actions; of number and the operations of arithmetic; of places in the navigable layout and their distances and directions from one another; and of geometrical forms and their length and angular relations. This chapter examines this knowledge hypothesis by considering each of its three claims: that infants' knowledge is guided by systems; that the systems are at the core of mature reasoning in these domains; and that these systems' computations give rise to knowledge. It reviews how investigating boundary conditions and signature limits allowed the discovery and exploration of these systems across ages, species, and cultures. It suggests that understanding the nature of infants' social reasoning abilities will require a similar effort. The ways such an approach could help to clarify current theories of human social cognitive development are discussed.
