In his theory of cognitive development, Jean Piaget proposed that children actively construct and modify their understanding of the world through the processes of assimilation and accommodation. They form schemas that help them organize their experiences. Progressing from the simplicity of the sensorimotor stage of the first two years, in which they develop object permanence, children move to more complex ways of thinking. In the pre-operational stage (about age 2 to about 6 or 7), they develop a theory of mind, but they are egocentric and unable to perform simple logical operations. At about age 7, they enter the concrete operational stage and are able to comprehend the principle of conservation. By about age 12, children enter the formal operational stage and can reason systematically.
Research supports the sequence Piaget proposed, but it also shows that young children are more capable, and their development more continuous, than he believed.
Lev Vygotsky's studies of child development focused on the ways a child's mind grows by interacting with the social environment. In his view, parents and caretakers provide temporary scaffolds enabling children to step to higher levels of learning.