There are a number of different stages of development that occur for infant. Of these,
intellecutal development, and motor development are the most prominant forms.
Until about the age of fifteen, children are not capable of reasoning like an adult. This is most obviously true for an infant between zero and twenty four months of age. The time of intellectual development between these two ages is known as the sensory motor period. The reflexive stage, which occurs between zero and two months, is when simple reflexes such as grasping and sucking are prominant. Primary circular reactions happen between two and four months. These are reflexive behaviors that occur in stereotyped repetition, such as opening and closing fingers repetitively. The repetition of change actions to reproduce interesting consequences, such as kicking one's feet to move a mobile suspended over the crib, is known as the second circular reactions. They occur between four and eight months.
That is nearly twice as long as the stage of primary circular reactions. Cooridnations of secondary reactions, occuring between eight and twelve months, is where responses become more coordinated into complex sequences. Actions take on an "intentional" characteristic. Examples of such would be an infance reaching to obtain a hidden object. Between twelve and eighteen months, tertiary circular reations take place. These are the discovery of new ways to produce a consequence or obtain a goal. Finally, invention of new means through mental combination occurs between eighteen and twenty four months. At this point, there is evedince of an internal representational system. Infants are able to symbolize the problem-solving sequence before acutally responding. These six steps are a part of Piaget's Stages of Cognative Development. Jean Piaget was a developmental biologist who devoted his life to observing and recording the development of infants, children, and adolescents.