There are four parts to the theory of cognitive development postulated by biologist Jean Piaget regarding the mental and psychological development of children. The stages are the schema, assimilation, accommodation and equilibrium. Piaget argues that early cognitive development involves processes based upon actions that later progress into changes in mental operation.
The first is the schema, which is both the mental and physical actions involved in obtaining, understanding and knowing. The second is about assimilation, which is the process of absorbing new information into our existing schema. The third is accommodation, which is a part of adaptation that involves changing or altering the existing schemas as a result of new information. The fourth is equilibration, which is trying to strike a balance between assimilation and accommodation.
The actual stages of cognitive development begin with the sensory-motor stage, which takes place in infancy. It's the stage where intelligence is demonstrated through motor activity and knowledge of the world is limited because it's based only on physical interactions. Next comes the pre-operational stage, which happens during the toddler and early childhood years. Language use begins in this stage, and memory and imagination are developed, but thinking is still done in a non-logical way. After that comes the concrete-operational stage of childhood. This stage goes through early adolescence, when operational thinking and the ability to perform classification tasks and order objects in a logical sequence develops. The child is capable at this point of problem solving. Finally comes the formal-operational stage, which goes into adulthood, where thought becomes more abstract and incorporates the principles of formal logic. The ability to generate abstract propositions, multiple hypotheses and their possible outcomes is evident. Thinking becomes less tied to concrete reality.