Piaget defined four stages of cognitive development: sensorimotor, preoperational, concrete operational, and formal operational. Piaget developed his theory while observing his own nephew and daughter. Piaget's theory is unique in that, before this time, children were treated just like tiny adults in terms of what they could be taught. Piaget's theory laid the groundwork for age-appropriate education and skill development in children.
In the sensorimotor stage, which lasts from birth to approximately two years of age, children learn that they can manipulate their environment. Children learn by tasting, touching, hearing, and looking. Children also develop object permanence, which means that they know an object continues to exist even if they can no longer see it.
In the preoperational stage, which lasts from two to seven years of age, children can be egocentric and sometimes feel little empathy. They can also use words and pictures to represent objects. This helps the child better explain themselves when talking about objects. They still struggle with abstract thinking.
In the concrete operational stage, which lasts from seven to eleven years of age, children begin to develop inductive logic. They also understand the concept of conservation—for example, a tall skinny cup can hold as much as a short fat cup, provided that they can hold the same amount of ounces. They also understand that their thoughts are unique to them and that their viewpoints are unique.
In the formal operational stage, which takes place from the age of twelve, children develop abstract thought and deductive logic. Children can also think clearly about hypothetical situations. Teenagers can also grasp political and moral questions more easily and debate them with others.
Piaget's theory was revolutionary in that he pointed out that children changed the way they thought and that their continued development was not only attached to the continued acquisition of knowledge. This was important because it implied that the brain was still developing in children all the way through what we would call adolescence today.