Initially, the baby makes sense of the world, primarily…

Initially, the baby makes sense of the world, primarily through his reflexes, his sensory perceptions, and his physical actions upon the world. After a year or two, he arrives at a "practical" or "sensorimotor" knowledge of the world of objects, as they exist in time and space. Equipped with this knowledge, he can make his way satisfactorily in his environment and can appreciate that an object continues to exist in space and time even when it is out of view. Next the toddler goes on to develop interiorized actions or mental operations. These are actions that can potentially be performed upon the world of objects; but, owing to a newly emerging capacity, these actions need only be performed cerebrally, within the head, perhaps through imagery. So, for example, to proceed from his destination to a familiar starting point, the child does not have to try out various routes: he can simply calculate that by reversing his steps, he will return to his origin. At the same time, the child also becomes capable of symbol use: now he can use various images or elements such as words, gestures, or pictures-to stand for real life objects in the world, and can become skilled in de-ploying various symbol systems, like language or drawing.

— from Time

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