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).