「誠」這一概念乃是中、日兩國儒者都非常重視的道德概念。在中國特別注重「誠」的，自不待言地當推朱子學；而在江戶日本特別重視「誠」的，首推山鹿素行(1622-1685)、伊藤仁齋(1627-1705)等人所提倡的古學。但兩者雖然同樣重視「誠」這一概念，但兩者所理解的「誠」概念的內含並不相同。本文透過分析、比較中國朱子學的「誠」概念與日本古學派學者山鹿素行的「誠」概念，除闡明兩者之間的異同，同時亦更進一步確認日本倫理思想的特色。 藉由本文的檢討，我們可以明白：中國朱子學式的「誠」與日本古學派的「誠」，兩者對「理」所抱持的態度恰為相反。簡言之，山鹿素行的「誠」概念是往脫離「理」的方向而加以開展；而中國朱子學的「誠」概念則是往趨向「理」的方向來發展。亦即，山鹿素行所理解的「誠」並不是以超越、抽象的「理」為基礎，而是以內在、具體的「不得已之人情」為基礎，而此一特點正反映出所謂：日本倫理思想的特色，乃在以「人情」為基礎的這一特色。

Cheng (誠) as a concept of morality is highly valued by Chinese and Japanese Confucianist. In China, it's undoubted that the thought of Zhu Xi valued Cheng greatly. And in Japan, Cheng was also paid much attention by Kogigaku (古學), which was a school that promoted by Yamago Soko (山鹿素行) and Ito Jinsai (伊藤仁齋). However they had different understanding of Cheng. In this paper, I compared the concept of Cheng in Zhu Xi's understanding with Yamagos' to explain the similarities and dissimilarities of them and to confirm the characteristic of Japanese ethics. We can see that Zhu Xi's understanding of Cheng and Kogigakus' have the different attitude toward Li (理) in this paper. Briefly, Yamago's understanding is removed from Li, but Zhu Xis' tended to it. It means that Yamago's understanding is not base on the abstract Li but the internal and specific emotion. And it also reflects that emotion is the characteristic of Japanese ethics.