Vygotsky, L. V. (1981)
The genesis of higher mental functions. In J.V. Wertsch (Ed.) The concept of activity in Soviet psychology. Armonk, NY: Sharpe.
The very mechanism underlying higher mental functions is a
copy from social interaction; all higher mental functions are
internalized social relationships. . . Even when we turn to mental
[internal] processes, their nature remains quasi-social. In their
own private sphere, human beings retain the functions of social
interaction (p. 164).