In Semantic and Conceptual Development, Frank Keil presents the first psychological investigation of the developing child's ontological knowledge. Building on previous philosophical work, Keil shows that ontological categories develop in a highly predictable progression. Moreover, Keil demonstrates that ontological development obeys a strong formal constraint on the relations among categories. Although there are many possible ontological systems, children appear to be inherently targeted to consider a system of only one sort.

Keil's results represent exactly the...