This is the stage when Infants begin to learn through sensory observation, and they gain control of their motor functions through activity, exploration and manipulation of the environment. From birth, biology and experience work together to produce learned behavior. As infants become more mobile, one action is built upon another action, forming new and more complex actions. Infants" spatial, visual, and tactile worlds expand during this period in which children actively interact with their environment and use previously learned behaviors. The critical achievement of this period is the development of object permanence. This is the indication that a child has the ability to understand that objects have an existence independent of the child's involvement with them. Infants learn to differentiate themselves from the world and are able to maintain a mental image of an object, even when it is not present and visible. At about 18 months, infants begin to develop mental symbols and to use words. This process is called symbolization. Infants are able to create a visual or mental image of an object to stand for or signify the real object. The attainment of object permanence marks the transition from the sensorimotor stage to the preoperational stage. During the stage of peoperational thought, children use language and symbols more extensively than in the sensorimotor stage. Children learn without the use of reasoning, therefore are unable to think logically or deductively. Children are able to name the object but they are unable to categorize or class these objects. Preopreational thought is midway between socialized adult thought and the completely autistic freudian unconscious. Events are also not linked by logic. In this stage, children begin to use language and drawings in more elaborate ways. From once using one word phrases they begin to use two word phrases. Children in this developmental stage are egocentric.
