He claimed that children at this age literally can't understand that others can see the world differently from them.

他认为这个年龄的儿童完全没有意识到,别人眼中的世界,可以与自己看到的世界有所不同

耶鲁公开课 - 心理学导论课程节选

And his discoveries that-- they did them in different ways at different ages led to the emergence of the Stage Theory.

他发现,儿童在不同的年龄拥有不同的行为方式,他根据这一发现提出了阶段理论

耶鲁公开课 - 心理学导论课程节选