Probably as a result of his training in biology, Piaget believed that intellectual development was the result of both genetic and environmental factors. This did not 'sit well' with the scientific thinking of the day - a time when Behaviorist theories purported that learning was a direct result of environmental stimuli. It would not be until much later that Piagets' theories would be recognized and valued - in the early 1920s and '30s he was perceived to be a radical. Piaget's theory stated that children moved through distinct stages in their development of knowledge and that the order of these stages were fixed - a child in an early stage could not suddenly 'jump' to a much later stage. It is during this stage that infants struggle to attain control of their bodies.
Newborns have simple reflexes, which they gradually learn to control in order to achieve some goal.
In other words, if a toy is hidden from view they quickly lose interest and will not search for the toy. The development of the concept of object permanence is one of the main achievements during the sensorimotor stage - this involves the ability to form a mental representation of the object and realizing that the object is still 'there' despite the fact that it has been hidden. Also, during this stage, the children begin to understand the concept of object constancy - that it is the same object regardless of distance, light, or different viewing angle for example. It is during this stage that language is rapidly acquired. Labels can now be given to objects and mental maps or 'schemas' are increasingly used.