古今《易》學家頗用「日光之有無」來解釋「陰」、「陽」二字的本義，論《易傳》則著意從「氣」的層次闡釋《易傳》宇宙論中的「陰」與「陽」之關係，不復考慮日光的問題。本文首先指出「易」原理即為日光的顯隱，「一天」由白天與黑夜循環一次而成，而白天與黑夜的轉變，即取決於太陽的顯現和隱沒。「一年」由春夏秋冬循環一次而成，而寒暑的往來，亦取決於日照的強與弱。地軸線以23.5度傾斜自轉，導致地球有四季亦即陰陽的變化。陰陽的遞嬗循環，氣候的寒燥循環，主要取決於「陰」的壯盛與否。《易》理以「陽」、「剛」為主不以「陰」、「柔」為主，期故在此。本文從科學原理分析「易」理「主陽」觀念的形成原因後，進一步認為卦爻辭已確立了陽大陰小、陽吉陰凶的觀念。其後《易傳》、董仲舒、荀爽、王弼、張載、朱熹等哲學家加以分析、演繹，都將乾坤、剛柔、陰陽的關係指向以「乾」、「剛」、「陽」為主的方向，對「易道主剛」概念作不同的發揮，包括陽貴陰賤、陰尊陰卑、陽主陰從、陽義陰利等思想。本文理念，雖似尖新，但也許可以替「易」的原理多提供一個思考的角度。

In the last few centuries, many Yijing scholars explained Dao (the Way) in terms of the interaction of yin and yang. They assumed that the two material forces are coequal and play the same role in the Yijing. But in the early Yijing tradition, yang is considered to be a more important force of change. The reason is the Yijing describes a solar-centered universe. In the Yijing text, the references to day and light, and the four seasons are actually reflections of the cyclical movement of sunlight on the north and south hemispheres. To describe the change in sunlight, yang means sunshine and yin means shade or coldness caused by the lack of sunlight. Either way, yang is the ultimate force that causes changes to take place. In this paper, I will demonstrate how this solar-based cosmology is presented in the Yijing and how it is developed into a system of thought by commentators such as Daong Zhonghu, Xun Shuang, Wang Bi, and Zhu Xi.