Some criticisms of this part of the theory are concepts such as "schema", "accommodation", "assimilation" and "equilibrium" are too vague. There is also no firm evidence that these concepts actually exist. Piaget observed and made only assumptions.
Piaget also believed that development occurred in four stages. He maintained that these stages were not missed and that children move through them in this order. He also said that these stages applied to all children regardless of their social or cultural norms.
The first stage, the sensori-motor stage, describes the first two years of an infants life. Schemas are based on sensory experiences and physical (motor) reactions and responses. Piaget further sub-divides this stage. The infant progresses through these stages, eventually reaching the ability to think symbolically and mentally. The child can now "think in their heads" before they act.
The second stage is named the pre-operational stage, lasts from 2 -7 years of age. Piaget reckoned, at this stage children demonstrate a tendency to think only from their own perspective. They cannot separate their own point of view from others or distinguish a separate reality. Piaget called this egocentrism. Another aspect of the pre-operational stage is the tendency to think animistically. Animistic thinking is the term used by Piaget which attributes life-like qualities to inanimate objects such as trees or the moon. For example, a child may believe that the moon follows them as they move. It's only later, with growth and development that they understand.
Piaget also believed at this stage, children haven't developed the ability of conservation, that is, children seem to have difficulties understanding that a change in the way something looks does not necessarily mean a change in other attributes such as number or quantity. For example, a child would be shown two beakers, one tall and thin, and the other shorter and wider, both filled with equal amounts of water.