伊川先生學說的重心在於儒家所肯定的道德生活與人倫的和諧，亦即重視修養過程與德性行為的實踐。然而他的形上理論根據則落實於他的宇宙論和人性論上面，亦即伊川所提出「性即理」的命題上，「理」屬於宇宙論的範圍，「性」屬於心性論的範圍，而伊川用一「即」字，將這兩個不同範圍的名詞，做若千程度關係的連接，這同時也引發了後人的爭論。本文針對一、伊川「理」的提出，他的定義和特色為何？二、伊川「性」的提出，他的定義和特色為何？三、伊川思想中「即」字的涵義為何？四、伊川「性即理」的理論基礎為何？五、伊川「性即理」的內涵與外延為何？有無限制？六、伊川「性即理」說的價值與影響為何？等六項議題，提出探討與論述。

　　　　　The core of Cheng Yi (1033---1107) (A famous scholar in the Sung Dynasty) 's theory is about the harmony of moral life and human-relation-principles that Confucianism approves, that is to say, emphasizing the process of putting self-cultivation and virtue into practice. Nevertheless, the basis of his metaphysics carries out in his cosmology and his theory of human nature, namely, it is about how Cheng Yi assigned a topic on "nature is reason." "Reason" belongs to the scope of cosmology and "nature" belongs to the range of the theory of human nature. Cheng Yi used the word "and" to connect these two different field of words to a certain degree. This action also arose plenty of controversies in later times. This essay is focus on six topics. The first part is about the definitions and characteristics that Cheng Yi pointed out for "reason." The second part is about the definitions and characteristics that Cheng Yi raised for "nature." The third part is talking about the meaning of the word "and" in Cheng Yi's point of view. The fourth part is about the basis of Cheng Yi' s "nature is reason" theory. The fifth part is about the connotation and the extension of Cheng Yi' s "nature is mason" theory. The last part is about the worth and influence of Cheng Yi's "nature is reason" theory. I will discuss the above-mentioned six topics in m essay.