Interaction with the culture is the agent of change allowing development of higher level thinking. Vygotsky concluded, based on studies, including studies of apes, that thought and speech have different genetic roots. Thought and language develop independently even in humans. When humans are around two years of age, however, thought and language join to initiate a new form. In humans, a prelinguistic phase in the development of thought and a preintellectual phase in the development of speech can be observed, When the two merge, thought becomes verbal, and speech becomes rational.
Egocentric speech develops into internal speech.
Vygotsky added frustrations and difficulties to Piaget's experiments. Words play a central part not only in the development of thought but in the historical growth of human consciousness as a whole. A word is a microcosm of human consciousness. Human beings are not limited to their biological inheritance, as other species are, but are born into an environment that is shaped by the activities of previous generations. Humans have the capacity to continue to develop intelligence.