题目：
简要回答：
描述皮亚杰提出的认知发展的四个阶段，并且说明每个阶段主要的发展性变化。
答案：
1, stage movement perception: the use of children in some of the original structure to deal with external objects.
感知运动阶段：儿童运用某种原初的格局来对待外部客体。
2, speech delivered before the stage: the child began to describe the symbol as an intermediary to the outside world.
前运演阶段：儿童开始以符号作为中介来描述外部世界。
3, Yun-specific phase of the speech: Children can carry out specific speech delivered at liaison with the concrete things, to carry out the logic of his delivery.
具体运演阶段：儿童能进行具体运演，在同具体事物联系的情况下，进行逻辑运演。
4, speech delivered in the form of stage: a proposition calculus.
形式运演阶段：处理命题演算。
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心理学Piaget'sfourstagesofcognitivedevelopment
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