Currently out of print, this is a classic by the famed Swiss Biologist and Child Psychologist, Jean Piaget. Piaget believed that intelligence is a form of adaptation, wherein knowledge is constructed by each individual through the two complementary processes of assimilation and accommodation. He theorized that as children interact with their physical and social environments, they organize information into groups of interrelated ideas called "schemes". When children encounter something new, they must either assimilate it into an existing scheme or create an entirely new scheme to deal with it (Wadsworth 1996).
Piaget also believed that intellectual development occurs in four distinct stages. The sensorimotor stage begins at birth, and lasts until the child is approximately two years old. At this stage, the child cannot form mental representations of objects that are outside his immediate view, so his intelligence develops through his motor interactions with his environment. The preoperational stage typically lasts until the child is 6 or 7. According to Piaget, this is the stage where true "thought" emerges. Preoperational children are able to make mental representations of unseen objects, but they cannot use deductive reasoning. The concrete operations stage follows, and lasts until the child is 11 or 12. Concrete operational children are able to use deductive reasoning, demonstrate conservation of number, and can differentiate their perspective from that of other people. Formal operations is the final stage. Its most salient feature is the ability to think abstractly.