Schoolers generally believe Cheng Yi's proposition that "the same source of substance (ti) and function (yong)" stem form the Huayan school, but neglect the fact that "ti" means equality and "yong" means the difference in the latter. The discussions between Cheng Yi and his students show us that he did not, in fact, know the unique meaning of the ti-yong system of the Huayan school. This paper draws the conclusion that Cheng's proposition came from Wang Bi's construction of root (ben) and terminal (mo), and the Cheng brothers' ideas of beyond the form and under the form.