魏晉南北朝時期,易學最主要的是鄭玄、王弼兩家。李鼎祚評騭兩家說:"鄭則多參天象,王乃全釋人事。"此語頗嫌武斷,問題的關鍵在于如何理解鄭玄所謂的"天象"及"天道"。有別于王弼抽象化的天道論述,鄭玄將天道與天象緊密關聯,建立了具體而形上的天道論述。相比于以體用模式以及按照時間生成順序來理解形上-形下結構,基于空間的意象來理解其實更符合其本義。形而上之天道,便是人被造之來源;形而下之器具,便是人展現自身創造性、參天地化育之結果。在這樣一種空間意識的理解中,"形上-形下"之分不是抽象與具體之別,而是本源與生成之分。天道因此在鄭玄那里成為其"政教"的本原,而以二十八宿、干支等體系來解釋或理解具體的天道,是鄭玄"爻辰說"這一象數體例的形上學來源和政治學目的。以此對照王弼的言象意理路,有助于揭示六朝時期鄭王兩家此消彼長的思想實質。

Zheng Xuan and Wang Bi were the most important commentators on the Yijing in the Six Dynasties period. Li Dingzuo has said that Zheng Xuan preferred the principles of Heaven while Wang Bi focused on human affairs. However,this judgment is questionable: we need to study what did Zheng Xuan meant by"the principles of Heaven". When he spoke of what is "beyond form or beneath form",he was trying to show a spatial meaning besides an ontological or temporal meaning. That is,Zheng Xuan developed a spatial metaphor. Heaven which was above was the originator of human beings,while the implements which were below were the production of human beings. Therefore "above-form"and "below-form"are not about the difference between the abstract and idiographic,but the relationship between the source and the current.Zhengxuan’s Yaochen-learning,which explained the hexagram diagrams using lunar mansions,can thus be understood more clearly. We can also distinguish Zheng and Wang Bi’s learning more clearly.