The intellect of children is fundamentally different from that of adults. It is this assumption that is central to Piaget's theory of human cognitive development and the mental processes that children undertake. Piaget declared that the function of intelligence and human processing is so that the human can adapt to their environment. He saw mental processes, especially intelligence, as apparatuses for interaction with the world. These processes allow children to learn and adapt to their changing environment in order to survive. Piaget demonstrated this particular theory with his many studies of children.

Piaget considered that there are certain processes that underlie all learning.

The two primary processes are:

1. organisation of experiences, and
adaptation to the environment.

He found that at birth, children have only a few reflexes such as sucking and grasping which allow them to adapt to the environment. It is simply a reflexive action for a new born baby to suck on anything that touches it's lips or grasp anything that brushes it's palm. In other words, children are not born with any ready-made mental abilities.

Organisation can first be seen in an infant soon after birth when, instead of sucking as just a reflexive action, it becomes an organised pattern. When the child is picked up, he or she appears to actually seek out the mother's breast. This act of sucking has become a specific action. Piaget called these well defined actions and other such actions 'schemes' (some writers refer to these as 'schemas'). In this instance, a scheme is a generalisation that the child has made based on similar instances of physical actions (Borland, Burnham & Walker,1989). Schemes allow us to stably organise actions that are similar. Boden describes Piaget's scheme as 'the structure that is common to many actions in similar or analogous circumstances'(1994, 23). Piaget went on to argue that cognitive growth in children's development processes results from the changes that occur as their schemes change and become more complex with age.

Piaget argued that schemes could change through three separate processes of adaptation: assimilation, accommodation and equilibrium.

assimilation - the process of incorporating new information into one's existing knowledge of the world.
accommodation - the process through which one changes their existing mental structures and schemes in order to accommodate new information.
3. equilibration - the process through which assimilation and accommodation is balanced.

These three processes combine together to facilitate developmental progress. Each time a young child reacts to a situation, it requires some assimilation and accommodation. Whether it be an old situation or a totally new one, the child cannot act without using information that it has already learned. If the action that a child has just undertaken does not fit with it's previous learning, a scheme change is required. Kagen & Segal (1968) use the example of a young boy and his toys. The boy is given a new toy - a magnet. Initially, the boy will try and assimilate this new toy into the schemes he has related to his old toys. He may try to bounce it like a ball, blow into it like a horn or shake it like a rattle. When he realises that this new toy has a different quality, he will accommodate this into his view of toys. The boy now knows that not all toys are designed to bounce, blow or shake, but some can attract metal.

Décalage means that one finds similar changes occurring in cognitive developments at different ages. Piaget actually talks about two kinds of décalage: horizontal and vertical:

Horizontal décalage:
When children start to know that the mass of an object remains the same whether it is rolled up into a ball or rolled out into a long sausage shape, then Piaget would say that they have understood that its mass remains invariant over transformations in its shape. Now, you and I also know that its weight will remain the same over these transformations in shape too. And we can appreciate that the structural characteristics of our knowledge about these two invariant properties is equivalent: in both cases, X (mass) and Y (weight) remain constant despite changes in Z (shape). But children generally come to understand the invariance of mass about two years before they understand the invariance of weight. The developmental changes that their two understandings go through follow the same course. And in both these cases, the notion of invariance is being worked out at a symbolic level: that is, it's knowledge of the same sort in both situations. And thus Piaget describes it as 'horizontal décalage.

Vertical décalage:
 is a similar notion, except that it refers to the mastery of a notion at different stages of intellectual functioning, and Piaget sees these changes as having the same structural properties on both occasions. For example, toddlers develop a sensorimotor map of where they live, and can get from one room of the house to another quite easily. They can also make detours, so that they can get to the kitchen, say, by a number of different routes. But, it will be a long time before they are able to draw a map of the house, before they can represent their knowledge symbolically. Piaget's view is that the structural properties of the processes whereby the child gains a sensorimotor knowledge or a representational knowledge of the relations between places are the same, irrespective of the medium in which that knowledge is constructed. And this is what he means by 'vertical décalage': the formal organisation of knowledge is the same irrespective of its medium.