Cheng Yi belongs to the daoxue (道學) school of the Northern Song dynasty, which recognized the World as rational, and as such viewed all beings as governed by li (理). However, he also raised the issue of a moral subject through a discussion of the tension between tianli (天理) and renyu (人欲). But how can an irrationality like human desire exist within the rational cosmos governed by li? Does that mean that the order of li is not thorough? Mou Zongsan argued that Cheng Yi's li exists without action. But this makes Cheng Yi's famous remark that there is only one source for both the substance and the function, and there is no gap between what is apparent and what is hidden (體用一源, 顯微無間), which argues for the rational order governing all phenomena, a failed thesis. Cheng Yi detours the ontological discourse by transforming the problem of irrational humans within a rational cosmos into a problem of emotions in moral subjects. This transformation played an important role in shifting the focus of Neo-Confucianism from cosmology and ontology to human emotions.
程頤站在以世界看成為合理的北宋傳統中。他主張世界中的所有存在者都受到理的支配。不過,他通過「天理」和「人欲」的對立來,同時主張倫理主體的自律性問題。如果是這樣,理支配之下的合理世界中,人欲等的不合理性,則從何來的呢?是不是理的支配不夠徹底?牟宗三先生主張程頤的理「只存有而不活動」。果真如此,程頤說的現象世界的合理性,即「體用一源,顯微無間」命題則不能成立嗎?他把合理世界與不合理人的問題,轉換為倫理主體的道德情感以及倫理選擇的問題,從而他回避一切存在論的難題。程頤之如此的轉回,把宋明理學的中心話題,從宇宙論或存在論轉換為道德心理學。