Born in Switzerland in 1896, Jean Piaget published his first paper at the age of ten and would later in life develop his four-stage theory on cognitive development. During the development of his theory, he recognized young children would learn new skills or schemas, and attempt to apply these schemas to new objects, what he called assimilation, or eventually learn to make changes to schemas for different objects, called accommodations. His four-stage theory defined the different phases humans go through in life as they continue to learn at higher levels of functioning. The first stage of development Piaget described is the sensorimotor stage, where from birth to 24 months, children rely on their basic immediate senses and stimulus in their presence. Towards the end of this stage is when children begin to develop object permanence, recognizing things can still exist when out of their sight. The second stage is the preoperational stage, lasting from ages of two to seven. During this stage, children can begin understanding how to effectively use symbolism in their words and actions to represent things that are not there. Their view of the world is typically egocentric during this stage, and they have a common issue of only being able to center on one specific part of a situation at one time; a common problem to describe this stage is the inability for a child to understand pouring liquid from a wider diameter container to a thinner and longer container, that the taller container is still holding the same amount of liquid despite the difference in the immediate appearance. The third stage is the concrete operations stage and lasts from seven to 11. During this stage, children start to form the ability to decenter and can make logical sense of the conservation of substance and reversibility of objects. It also during this stage, children can begin to correctly perform classification and seriation of items. The final stage begins at around 12 through adulthood. The notable change at this stage is the ability to perform abstract reasoning.
