Jean Piaget was a Swiss philosopher that lived from 1896 to 1980. He had an expansive career researching the human development. His theory is one of constructivist. This means that he felt we (humans) construct our ability through self motivated action in the world. We are driven to explore and by doing so we learn. If we progress through all 4 stages we become fully functional and happy adults. If we get stuck in one stage or regress we lose our ability to function at the highest level inherent to us as humans.

The stages are divided into sensorimotor period (0-2 years), pre-operational (2-7 years), concrete operational (7-11 years) and formal operational (12 + years). Not everyone makes it past the pre-operational or concrete operational stage. That is why we all know the 35 year old man that still lives at home and mom does everything for him. To be a constructive and productive adult you MUST reach the formal operational stage.

Pre-operational stage: Babies are born with a set of congenital reflexes that help them to function in a busy world. These reflexes drive them to explore. At first a baby explores with his/her mouth (the sensorimotor phase). Below is a chart excerpted from Wikipedia on Piaget's theory:

This oral phase explains why a baby and toddler puts absolutely everything in their mouth. I see parents all the time slap their child's hand and say, "Don't put that in your mouth". Yes, it is frustrating. Yes, you should be sure they don't choke on small objects or get germs from dirty objects. But knowing that this is a normal phase of exploration and development helps you to tolerate the behavior a bit more. You begin to see it in a different light. Why won't your 2 year old go down the slide like everyone else. S/he stands there terrified. Well, maybe the other children have not reached the "beginning of symbolic representation stage". They are not at the point of thinking, "Hmmm. That appears rather dangerous". Your child is not being a wuss, he is being thoughtful and is actually quite progressive!

Pre-operational period - This stage lasts from roughly 2 - 7 years of age. This is the period when a child develops sparse and somewhat illogical thought processes. They learn to represent objects by images and words. This leads to a lot of pretend play. In the first part of this period (age 2-4) a child will assign human attributes to inanimate objects. This is when a teddy bear or security blanket becomes highly important. Sometimes an imaginery friend comes to live with you. By 4-7 years the child becomes intuitive. They use their brain to solve problems but don't know how they reached those conclusions. There is a reason why geometry is not introduced until later in school. You can tell what age a child is by knowing what math problems they bring home. It's fascinating. Mathematics teachers must have studied a lot of psychology before setting up their programs.

Concrete operational - This period occurs from 7 to 11 years. This is when kids are really fun. They are like mini-adults but much less flexible. Of course, I know a lot of adults stuck in this stage eternally. The child can think logically, classify items, and sequence information. They can follow complicated directions. However, everything is black and white to them. There is no way wiggle room. That is why your 9 year old reminds you that "I thought you weren't supposed to talk on the cell phone in the car, Mommy. Isn't that against the law?" "Mommy, why are you having another cookie? Isn't the rule only one cookie after dinner?" It can drive you nuts, but when you realize why they do it, you'll be proud! "Oh, Love. You are in the concrete operational stage. Isn't that fantastic??"

Formal operational - This is the final stage that many of us never reach. It is the goal of all pediatricians and parents for their patients and children to reach this stage. It is the point of abstract reasoning and thinking outside the box. This is when your child begins to ask questions about what you have told them. They realize that you don't know everything. In fact you are wrong about a lot of things. S/he will begin to challenge authority and test his/her limits. Once again, this is a normal stage of child development and a very important part of becoming an adult.