Swiss psychologist Jean Piaget (1896-1980) is arguably the most influential individual in the field of developmental psychology. Piaget's research undeniably laid the foundation for this scientific study of the psychological changes that are undergone by children as they mature into adulthood.
Curious about the reasons why children of different ages make different "kinds" of mistakes when they are learning, Piaget began to observe children of different ages, resulting in his theory that as children biologically mature; they pass through four stages of cognition. In layman's terms, cognition is the act of knowing and the process through which this knowledge is acquired and problems are solved.
Piaget's four chronological stages can each be distinguished by different types of thinking. In addition, each of these stages builds on the previous years and levels of understanding. Piaget argued that human beings develop psychologically by organizing stimuli into schemes, or clusters of knowledge that are then used to comprehend and decipher information. Throughout development, children will organize the stimuli they are exposed to through two distinct processes: assimilation and accommodation.
Assimilation is the process of absorbing new information. When they gather new information, children will attempt to place this knowledge into existing schemas. Accommodation, on the other hand, is the process of changing these existing schemas in order to better interpret the new information. However, both assimilation and accommodation are used differently at different stages of biological development.
For example, from birth until two years of age, children experience the world through actions. They touch, look, and grab. Piaget called this stage the sensorimotor stage because children learned to create new schema by observing and interacting with their environments.
From two-years-old to six-years-old, children begin to accommodate their existing schemas by acquiring language, which allows them to represent the things they touched in the previous stage with words and images. This is known as the preoperational phase. While children might begin to strengthen their language skills, they have yet to develop logical reasoning. Images and words are arbitrarily connected, and they are unable to think abstractly.
From seven to eleven years of age, children begin the concrete operational stage. During this period, they are able to think logically about concrete events that have taken place. They begin to understand the concept of cause and effect, and can perform mathematical operations which are usually taught with visual aids. Children in this phase are more likely to understand concepts like one apple plus two apples equal three apples, rather than more abstract mathematical concepts.
From twelve to adulthood, human beings are then considered within the formal operational stage. Not only are they capable of fully assimilating and accommodating knowledge into their schemas, they can also think abstractly, using abstract logic and reason.
Although Piaget's methods and research have been highly criticized in the field of psychology, his observations have clearly had a massive impact on the way that we understand human cognitive development. Many critics disagree with Piaget's belief that cognitive development is a solitary process, which individuals can only achieve by interacting with the world; however, it seems likely, to me at least, that the initial phases of cognitive development, such as the sensorimotor stage, are extremely solitary. No matter how much parents can try to "teach" their babies through language, they will usually tend to learn certain things on their own.