Constructivist views Vygotsky's social interactionist theory belongs to a broader theoretical perspective, called the constructivist view-that people construct knowledge from their experiences and through interactions with their environment. Another very influential theory, called the cognitivist theory also belongs to this constructivist view. The key theorist for the constructivist view is Piaget. There are many similarities between Vygotsky and Piaget's theories. However, their ideas also differ from each other in many aspects. Piaget believes cognition (the mental process whereby knowledge is acquired and understood; essentially, what we know as 'thinking'/'thought') happens before learning. He believes that learning progresses through different stages: from concrete and action-based, to more abstract, logical and mental activities. For example, to learn the sentence, 'The ball is gone', a child needs to touch the object or observe the situation to understand the meaning. Gradually, the child can develop a mental image of it without seeing it. Piaget uses key concepts such as schema, assimilation and accommodation to describe the cognitive development process.