Lev Vygotsky
Language starts as a tool external to the child used for social interaction. The child guides its behavior by using a kind of self-talk or "thinking out loud". Initially, self-talk is very much a tool of social interaction and this tapers to negligible levels when the child is alone. Gradually, self-talk is used more as a tool for self-directed and self-regulating behavior. Because it gets internalized, self-talk is no longer present when the child starts school. Self-talk "develops along a rising not a declining, curve; it goes through an evolution, not an involution. In the end, it becomes inner speech".p57