Lev Vygotsky
Perhaps Vygotsky's most important work was the inter-relationship of language development and thought. This establishes the connection between speech and the development of mental concepts and cognitive awareness. Vygotsky described inner speech as being qualitatively different from normal (external) speech. Although Vygotsky believed inner speech developed from external speech by a gradual process, with younger children only really able to "think out loud".