Vygotsky accepted the idea of "Sub'ektnost" and relied on that in his Cultural-Historical theory. Vygotsky points out: An infant is a "sub'ekt" of development" (Vygotsky, 1982, V. 2, p. 281). He emphasized that culture is not just poured into the child: on the contrary, the child actively enters into culture, and commandeering culture elements which he needs, taking them from the outside and internalizing them.

"Dejatelnost" means a process of active and purposeful treatment of the environment, the outward activity. This was the main concept in Leontiev theory. He based on the Vygotsky's idea that human mental functions are structured in the process of social interaction, in the process of outward activity (Dejatelnost), and believed that analysis of Dejatelnost is the only way to understand Psyche.

Leontiev's Activity Theory

The theory of Leontiev was based on a combination of ideas of Rubinstein and Vygotsky. Vygotsky is mostly known outside Russia for his concept of the zone of proximal development. But his main contribution is a more general theory of internalization, known also as the socio-cultural historical theory. According to this theory the main part of human psychic set-up is formed in the course of socialization through interaction with other people and operating with tools. So first a psychic function is structured in the process of outward activity and then it is internalized.

For example, oral speech originates when a baby, initially lacking command of language, is involved in dialogue with his mother who is talking with the baby while she is attending his needs. In the course of the interaction with the mother, elements of the dialogue, words and phrases are internalized by the baby, and form the basis of endophasia and verbal thinking.

According to this theory, human psyche development is socio-culturally – historically specific. Development of human psychic functions is mediated by culture tools, which makes it different from the development of other living species. Human development, as Vygotsky stated, "switches from the natural path of development to the cultural one" when the "ignition" of the latter occurs in the course of socialization.

Leontiev developed the ideas of his predecessors in his own way, so that neither Vygotsky nor Rubinstein fully agreed with his reasoning, so that they both had perpetual discussions with Leontiev and even incurred personal enmity. So Leontiev's theory is justly considered to be an original one, not a mere derivative from the two above mentioned.