Vygotsky thinks that higher mental functions have social origins. Language is a system of signs that the individual needs in order to interact with the environment and only afterwards it is interiorized and can be utilized to express thought. The meaning of a word is initially a purely emotional fact. Only with time it will acquire a precise reference to an object and then an abstract meaning. Child development is a sequence of stages that lead to the transformation of an interpersonal process into an intrapersonal process.
Children think by memorizing, while adults memorize by thinking.
In children something is memorized, in adults the individual memorizes something. In the former case a link is created because of the simultaneous occurrence of two stimuli. In the latter case the individual creates that link. Remembering is transformed into an external activity. Humans are then able to influence their relation with the environment and through that environment change their own behavior.