The stages of cognitive development by Jean Piaget

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Jean Piaget began his career as a biologist -- specifically, a malacologist. But his interest in science and the history of science soon overtook his interest in snails and clams. He developed the idea of stages of cognitive development which constitute a lasting contribution to psychology.

The sensorimotor stage
The first stage, to which we have already referred, is the sensorimotor stage. It lasts from birth to about two years old. As the name implies, the infant uses senses and motor abilities to understand the world, beginning with reflexes and ending with complex combinations of sensorimotor skills.
Between one and four months, the child works on primary circular reactions -- just an action of his own which serves as a stimulus to which it responds with the same action, and around and around we go. For example, the baby may suck her thumb. That feels good, so she sucks some more... Or she may blow a bubble. That's interesting so I'll do it again....
Between four and 12 months, the infant turns to secondary circular reactions, which involve an act that extends out to the environment: She may squeeze a rubber duckie. It goes "quack." That's great, so do it again, and again, and again. She is learning "procedures that make interesting things last."

At this point, other things begin to show up as well. For example, babies become ticklish, although they must be aware that someone else is tickling them or it won't work. And they begin to develop object permanence. This is the ability to recognize that, just because you can't see something doesn't mean it's gone! Younger infants seem to function by an "out of sight, out of mind" schema. Older infants remember, and may even try to find things they can no longer see.
Between 12 months and 24 months, the child works on tertiary circular reactions. They consist of the same "making interesting things last" cycle, except with constant variation. I hit the drum with the stick -- rat-tat-tat-tat. I hit the block with the stick -- thump-thump. I hit the table with the stick -- clunk-clunk. I hit daddy with the stick -- ouch-ouch. This kind of active experimentation is best seen during feeding time, when discovering new and interesting ways of throwing your spoon, dish, and food.

Around one and a half, the child is clearly developing mental representation, that is, the ability to hold an image in their mind for a period beyond the immediate experience. For example, they can engage in deferred imitation, such as throwing a tantrum after seeing one an hour ago. They can use mental combinations to solve simple problems, such as putting down a toy in order to open a door. And they get good at pretending. Instead of using dollies essentially as something to sit at, suck on, or throw, now the child will sing to it, tuck it into bed, and so on.

Preoperational stage
The preoperational stage lasts from about two to about seven years old. Now that the child has mental representations and is able to pretend, it is a short step to the use of symbols.

A symbol is a thing that represents something else. A drawing, a written word, or a spoken word comes to be ...