Vygotsky's theory views human development as a socio-genetic process by which children gain mastery over cultural tools and signs in the course of interacting with others, especially with their adults and peers in their environments. This process of interaction between the child and a more competent adult is said to effect development if the interaction occurs within the child's zone of proximal development. Initially, the adult interacting with child assumes most of the responsibilities or guiding the problem solving, but gradually this responsibility transfers to the child. Language is a primary form of interaction through which adults transmit to the child the rich body of knowledge that exists in the culture. As learning progresses, the child's own language comes to use as his or her main means of intellectual adaptation. Eventually, children can use internal language to direct their own behaviour and this transition is known as internalisation.