Jean Piaget

"The child who cannot conserve is bound by the immediate perceptual field."

"Animism is not a sign of magical thinking but rather an understandable confusion about the boundaries of life."

"The child attributes life to things that move, a reasonable inference from limited experience."

"Irreversibility of thought characterizes the pre-operational stage of development."

"Once the child grasps reversibility, he understands that operations can be undone."

"Seriation is the ability to arrange objects in logical sequences, a key cognitive achievement."

"Classification requires the simultaneous consideration of similarities and differences."

"Number is not a property of objects but a logical invention of the mind."

"The child constructs number through coordinating the operations of seriation and classification."

"Mathematics is not discovered but invented by the human mind in its dealings with experience."

"Logical-mathematical knowledge is constructed through the child's own activity."

"Physical knowledge comes from the properties of objects themselves, not the child's actions."

"Social-conventional knowledge is transmitted by the culture and cannot be invented by the individual."

"The three types of knowledge interact in the developing mind."

"Schema is the basic unit of cognitive organization."

"Schemas are not static but continually modified through experience."

"The infant's world begins with reflexes and gradually becomes more sophisticated."

"Sensory experience is the foundation upon which all later cognition is built."

"The baby learns that the world exists when he is not looking at it."

"Object permanence is a revolutionary understanding that the world has continuity."

"The sensorimotor infant gradually understands cause and effect through repeated experience."

"Imitation becomes increasingly sophisticated as the child develops."

"Deferred imitation shows that the child can represent absent models."

"Symbolic play allows the child to represent reality in new and creative ways."

"Games with rules mark the transition from solitary play to social play."

"The child's morality evolves from heteronomous to autonomous as he develops."

"Young children believe rules are fixed and absolute, unchangeable by mutual agreement."

"As children mature, they understand that rules are cooperative agreements that can be modified."

"Moral development requires social interaction and the opportunity to consider others' perspectives."

"The child who has never had to justify his actions to others cannot develop moral reasoning."