His theory was based on the qualitative development of knowledge. He considered cognitive structures development as a differentiation of biological regulations. There are four stages of Piaget's theory.
1. Sensorimotor Stage: from birth to age two. The children experience the world through movement and their five senses.
2. Preoperational Stage: Piaget's second stage, the pre-operational stage, starts when the child begins to learn to speak at age two and lasts up until the age of seven.
3. Concrete Operational Stage: from ages seven to eleven. Children can now conserve and think logically (they understand reversibility) but are limited to what they can physically manipulate.
4. Formal Operational Stage: from age eleven to sixteen and onwards (development of abstract reasoning). Children develop abstract thought and can easily conserve and think logically in their mind.
Also developed the Theory of Schemas, the building blocks of knowledge.