Cognitive development is a field of study in neuroscience and psychology focusing on a child's development in terms of information processing, conceptual resources, perceptual skill, language learning, and other aspects of brain development and cognitive psychology compared to an adult's point of view. In other words, cognitive development is the emergence of the ability to think and understand. A large portion of research has gone into understanding how a child imagines the world. Jean Piaget was a major force in the establishment of this field, forming his "theory of cognitive development". Piaget proposed four stages of cognitive development: the sensorimotor, preoperational, concrete operational and formal operational period. Many of his theoretical claims have since fallen out of favor. However, his description of the more prominent changes in cognition with age (e.g., that it moves from being dependent on actions and perception in infancy to an understanding of the more observable aspects of reality in childhood to capturing the underlying abstract rules and principles in adolescence) is generally still accepted today. Perhaps equally importantly, Piaget identified and described many cognitive changes that must be explained, such as object permanence in infancy and the understanding of logical relations and cause-effect reasoning in school age children. The many phenomena he described still attract the interest of many current researchers.