在宋明理學中，主宰宇宙生化的本體乃是「理」或「天理」。「性」字原就「個體」之存在說，後來亦指宇宙本體即「性體」。伊川乃提出「性即理」這一劃時代的理論，成為宋明儒學的基本教義。另一方面，因宇宙萬物的存在基質是「氣」，理學在討論理、氣的關係上，經朱子的發展，造成理氣二元論的性格過於明顯。明代氣論學者羅整庵、王廷相都力主「天地一氣」，然而，卻只承認「氣」才能終古常在，反對「性」無生滅之說；則「性體」義難以成立。本文指出，船山亦即氣言天，其所謂「氣」，乃理、氣「合兩端於一體」的本體。「理」意指本體一「氣」本身乃是一固有主宰生化之「性」的實體，氣終古常在，「性（理）」亦然。渾然一氣，純然一理，亦純然一性，天地本體一「氣」即一「理」體、一「性」體；氣之「性」的義涵，乃「健順」之德、知能之用。最後，在這些問題的基礎上，從四個方面，比較船山與宋明大儒之「性體」觀念的不同，以說明其理論價值。

In the doctrine of the Confucian scholars of the Song-Ming Dynasty, the master asserted that Li (the laws underlying all things and events) constituted the universal substance. The word of 'Hsing (Nature)' originally indicated an individual being, but its significance finally became the universal substance (prima materia). So Cheng I-Chuan brought up the epochal idea of 'Nature is Li (Principle)'. This concept became an essential aspect of the teaching during the development process of the Song-Ming Dynasty Confucianism. In the other hand, Chi had all along been treated as the ultimate element that made up all things. What was the relationship between Li and Chi? As developed by Chu Hsi, it would lead to a conclusion that implies a dualistic relationship between Li and Chi. The Chi theories of Wang Ting-Hsiang and Lo chin-shun in the Ming Dynasty both asserted that the universal substance is the Chi of the ultimate 'One' and that Chi must partake in eternal being, thus opposing the view that 'Hsing (Nature)' can not become extinct. In their theory, the concept of 'Hsing-Ti (Moral Creative Reality)' could not be established. Wang Chuan-Shan asserted that Li and Chi were two aspects opposite to each other and parts of an organic whole. In Wang's theory, Nature is Principle too. In Wang's concept of 'Li (Principle)', Chi itself, as universal substance, constitutes an entity that overrules all of the laws of birth, growth, decay and death, governing all things. The contents of Chi's 'Hsing (Nature)' were the virtues of Jiahn (creativity), Shuhn (receptivity) and Chih (knowing and enlightening), and Neng (accomplishment). The entity of Chi has eternal being, as does the entity of Li (Hsin).