Vygotsky took hard the collapse of the unity of adherents of socio-cultural theory. In a private letter to Leontiev from August 7th 1933 (Leontiev et al., 2005) Vygotsky wrote: "…as a matter of fact, your definitive departure to Kharkov is our heavy, crushing failure…"

Regarding the difference in their views, it was perhaps best summarized by P. J. Galperin, a younger colleague of Vygotsky and Leontiev, who, remembering the Kharkov period, stressed that Leontiev's doctrine "has led to essential change in the focus of research: Vygotsky focused on the influence of the higher (social) mental functions on the development of natural mental functions and practical activities of a child, while Leontiev focused on the leading part of external instrumental activity in the development of mental activity, in consciousness development" (Galperin, 1983, p. 241)[21].

Vygotsky points out that external activity is inseparable from inner mental activity. Any external operation is the result of a play of internal recourses. Explaining the importance of the "social situation of development", Vygotsky emphasizes that an infant is an active participant of the latter. First, growing, the child changes the situation by his actions. Second, his perception of the social situation is transformed under the influence of his internal attitudes and life plans. Vygotsky's fundamental conclusion is that the "main path" of development is not a gradual socialization introduced into the child from outside, but a gradual individualization that occurs on the basis of the internal activity of the child.

It should be noted also that in the letter which Vygotsky wrote to Leontiev, cited above, there are the words: "The inner bears the impact of the outer, but, of course, the former is not determined by the latter entirely". This seems much in tune with the Rubinstein disagreement with Leontiev.

Was there a disagreement between A. N. Leontiev and his predecessors? A. N. Leontiev and S. L. Rubinstein

After the repressions of the 1930s, strange as it might seem, World War II turned out to be a time of most fruitful and intensive development for Soviet psychology. The necessity to take part in the struggle against fascism which united all Soviet people, the need for psychological knowledge to be applied for war purposes, as well as for rehabilitation of wounded soldiers, and a great amount of unique empirical data – all these contributed to facilitate the development of psychology. New advanced theoretical approaches appeared such as the neuropsychological theory of Luria based on ideas of Vygotsky, and a theory of individual development in adulthood by B. G. Ananiev.