Assimilation was originally a biological concept, which refers to the process of turning food into a part of itself through digestion and absorption after an organism ingests food. Piaget borrowed this term from psychology to describe "integrating external elements into a forming or already formed structure" (Piaget, B. Geldet, 1980). In Piaget's view, psychology, like physiology, has a process of absorbing external stimuli and making them a part of itself. The difference is that the change table involved is not physiological but functional. With the development of personal knowledge, assimilation has gone through the following three forms: