Lev Vygotsky was a Russian psychologist known for his "social development theory" of learning. This theory proposes that social interaction and culture greatly influence cognitive development. Vygotsky believed that cognitive processes like language, thought, and reasoning develop through social interactions and with assistance from others with more advanced skills, or "more knowledgeable others." A key concept from Vygotsky is the "zone of proximal development," which refers to tasks a child can complete with guidance but not independently, representing the next stage of development. Vygotsky's theories emphasize the important role of social and cultural factors in learning.