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Cognition refers to all the mental activities associated with thinking, knowing, remembering, and communicating. From the time we first open our eyes to perceive the world, how does our mind develop from there? Developmental psychologist Jean Piaget spent his life searching for the answers to such questions. Where others saw childish mistakes, Piaget saw intelligence at work.

Thanks partly to his contributions, we now understand that a child's mind is not a miniature model of an adult's. Children reason differently and are incapable of adult logic. And he noticed certain patterns of mistakes in intelligence tests, and then his own tests, that indicated differences in reasoning at what he ultimately saw as different stages of development.

As a child advances to each new stage, his thinking is qualitatively different from that of the previous stage. Piaget's core idea is that the driving force behind our intellectual progression is an unceasing struggle to make sense of our experiences. To this end, the maturing brain builds schemas, concepts or mental molds into which we pour our experiences.

But how do we use these schemas? Piaget called these processes assimilation and accommodation. In assimilation, the child interprets new experiences in terms of the things he or she already knows, into existing schemas. For example, a toddler might call anything with four wheels a car. However, when the child encounters new information, he or she must adjust the current understanding through accommodation. Here, the child cognitively makes distinctions between schemas. So the same toddler might learn that trucks also have four wheels, and have to refine the original scheme of cars to fit this new information.

Piaget's thought that we continually alter our schemas through our life span and that these stages could be grouped into predictable patterns.

The four major stages are sensorimotor, preoperational, concrete operational, and formal operation.

In the sensorimotor stage, from birth to nearly age two, babies take in the world through their senses and actions. Very young babies seem to live in the present. Out of sight is out of mind. That is, infants lack object permanence, or the awareness that things continue to exist even when they're out of sight.

Infants have a receptive vocabulary that can be quite large by the end of the first year. So at the very beginning, it's recognizing sights and sounds and making sense of everything. If you see a baby, as soon as they can move their head, they're organizing their world and trying to actively be part of it.

One of the classic demonstrations of that was the A not B error, in which you hide an object in front a location, and then move that object right in front the baby. And people go, look baby, and they shake it and everything to another second location. And the baby makes a classic error.

The baby acts as if— According to Piaget, the baby has no longer any representation of that object, and so, doesn't grasp for the object. It's as if the object is now out of sight, and it's out of mind.

Following the sensorimotor stage, Piaget believed that until ages six or seven, children are in a preoperational stage, too young to perform mental operations.

So for example, if I physically change the appearance of a glass of water by taking from a beaker and pouring into long cylinder, you know that the amount of water, unless I spilled some, hasn't changed at all.

Now, which glass has more juice, this one or this one?

This one.

What's important to the preoperational child is the change in physical appearance.

That has more.

Why do you think it has more?

Because that's taller than the other.

You and I know that there's neither more nor less in there. At the end of the preoperational period, the child's articulates that. But until the child gets there, what he or she says is, there's more because it's higher.

Piaget called this concept conservation, the principle that the properties of the substance remain the same, even if it changes shape.

Drink a lot of juice.

Additionally, Piaget contended that preschool children are egocentric. They have difficulty perceiving things from another's point of view. Though egocentrism persists, they slowly develop an ability to infer other's mental states when they begin forming a theory of mind. They seek to understand the perspective of others, and begin to interact by teasing, emphasizing, and persuading.

Then, according to Piaget, by about six or seven years old children enter the concrete operational stage, at which point they become capable of true logical thought.

One thing to remember as we talk about concrete and informal, is they're getting more and more like us. So the differences aren't as stark, but they're still there.

Now which glass has more juice? This one, this one, or are they the same?

This one.

Why do you think that one has more juice?

Because there's a little left.

If you think about situations you've been in, you can capture what concrete intelligence is. It's a focus on the present, on the here and now.

We have the same amount.

We have the same.

By age 12, our reasoning expands from purely concrete to encompass abstract thinking. As children approach adolescence, many become capable of solving hypothetical problems and deducing consequences. If this, then that.

Where does Piaget's work stand today?

There's a tremendous respect for Piaget for founding the field, and for showing why knowledge has to be built up. And we're able to see Piaget as foundational in the field and realize that if we focus on the specifics of when such and such happens, we're missing the big point. And the big point was, in my opinion, the children are create their own development. That they're actively engaged in making sense of the world.

And I think that is just absolutely true, and in fact, we're trying to make education of that form. That it allows the child to construct actively the meanings of things like number in mathematics, whatever it might be, science— So once we got past the first idea— well children couldn't do this because Piaget said they couldn't do it. Then, children could do anything because Piaget was wrong. And now it's like, how do we teach to a way that makes sense of the children's mind and allows them to actively engage and construct their own meaning? And so it's still very interesting from an adult cognitive point of view, because it sort of shows how the mind is made.

Today we are layering new findings atop Piaget's foundational work as we seek to understand the development of our cognition, from birth to adulthood.