Lev Vygotsky
Speaking has thus developed along two lines, the line of social communication and the line of inner speech, by which the child mediates and regulates their activity through their thoughts. This is not to say that thinking cannot take place without language, but rather it is mediated by language, and so develops to a much higher level of sophistication. Just as the birthday cake as a sign provides much deeper meaning than its physical properties allow, inner speech as a sign provides much deeper meaning than the lower psychological functions would otherwise allow.