熊十力畢生倡導「體用不二」論，認為體用關係不即不離，在《新唯識論》中，強調「攝用歸體」，肯定本體真實，而至《乾坤衍》，則主「攝體歸用」，肯定現象真實。此一思想轉變過程，仍是在其中心理念下之內部修正，並非又另創一說，且益見其翕闢成變而即用識體之「體用不二」論之演變關鍵及其意義所在。 熊氏既主「攝體歸用」，完全著重於用上，由用以見體，於大用流行而識得本體，方成其為一內聖與外王亦不二之「體用不二」論。至其意義，即直捷以萬物為主，融通本體論、宇宙論與人生論，落實於此現象界中，既深符孔子《大易》開物成務、裁成輔相之旨，又彰顯其乃一活生生的實存而有之生活世界。

Hsiung Shih-li devotes his lifetime to advocating the theory of “unity in essence and action” which he finds the subtle relationship between the two. In Hsin Wei Shih Lun, he emphasizes the “Metaphysical Theory” (of integrating action in essence) and affirms the reality of ontology; however, in Ch’ien K’un Yen, he reverses his view on the theory and affirms the reality of phenomenon. This change is resulted from the inner modification of the central idea rather than the creation of another theory; hence, it is clear to see the key factor of the evolution and the significance of “unity in essence and action” theory. Hsiung promotes the “Metaphysical Theory” (of integrating essence in action) with the entire focus on action. His theory of “unity in essence and action” is completed in attempt to foreground the meaning of entity through action and thus recognize the ontology in great prevalence. The significance of the theory lies on universal beings. Through the incorporation of ontology, cosmology and life theory, the idea is implemented in the phenomenal world, which not only accords with the practical values of Confucius’ Da-Yi , but also manifests the substantial existence of the living world.