To make the mechanism under complex cognitive skills such as readings and calculation understandable,core knowledge systems are suggested as component cognitive systems with a long ontogenetic and phylogenetichistory which children and adults use as building blocks to construct new cognitive abilities. Studies of humaninfants suggest that core knowledge systems are mechanisms for representing and reasoning about particular kinds of ecologically important entities and events. Core knowledge systems are limited in a number of ways: They aredomain specific, task specific, and encapsulated. Research on older children and adults suggests that coreknowledge systems continue to exist and they serving as building blocks for the development of new cognitive skills. A single case study of core knowledge and cognitive development centering on the domain of number is presented in this article explaining how core knowledge system work. From research on core knowledge, some indications arise. First, human infants and other animals are also available for cognitive studies because they both exhibit many of the cognitive systems serve as cognitive skills’ building blocks. Second, comparison as a basiccognitive operation needs more studies on it. Third, research on conceptual formation and conceptual structure willhave some ideas from the combination of study on core knowledge system and psychological essentialism.