Interiorization (French interiorisation — transition from outside to inside, from Latin interior — internal) — the formation of internal structures of the human psyche through the assimilation of external social activities, the appropriation of life experience, the formation of mental functions and development in general. Any complex action, before becoming the property of the mind, must be implemented outside. Thanks to internalization, we can talk about ourselves and actually think without disturbing others. The term was first introduced by L. S. Vygotsky.
According to L. S. Vygotsky, any form of the human psyche is initially formed as an external, social form of communication between people, as labor or other activity, and only then, as a result of internalization, becomes a component of the human psyche. According to Vygotsky, each cultural form of behavior initially arises as a form of cooperation with other people, as an imitation of another or an appeal to an adult; only at the next step does this form become an individual function of the child himself. In its very essence, the cultural form of behavior is the result of «growing» the initially external, social, interpsychic form of behavior. Initially, the child’s behavior is a social form of cooperation with an adult. This cooperation, mediated by cultural tools and signs, initially appears in an external form.
The internalization of any skill occurs in three stages. At first, the action takes place only in the joint activity of the child and the adult, at the second stage — next to the adult, who performs the function of a hint and reminder, and only at the third stage the necessary actions are performed independently.
Subsequently, the process of internalization was studied by P. Ya Galperin and formed the basis for the systematically phased formation of mental actions and concepts.
External actions are never completely transferred to the internal plan. Only their tentative part is subjected to internalization. P.Ya.Galperin in this regard notes that «not all actions are completely transferred to the mental plan» [59, p.153]. Thus, writing in its executive, motor part always remains an external action. The orienting part of this action, as the child masters it, is reduced and goes inward, i.e. is interiorized. Among the actions that go entirely into the internal plan are many mathematical actions, for example, the action of counting.