五行中，「火」的來源最神祕，其他四行雖與天界有聯繫，但其存在根據都在大地。「火」的來源與依據卻被認為來自天界，尤其是太陽與「大火」此一恒星。太陽、月亮與星辰的運行有固定的軌道，軌道是歲、月、日等時間向度的來源，時間則是一切經驗成立的基礎，或者是康德 (Immanuel Kant, 1724-1804) 所說的「形式」。三光之中，「太陽」提供的熱能被視為「天火」，「天火」用以敬獻神祇。「火」也帶來熟食，熟食是文明之始，也是秩序之始，由「火」引燃的「天火」、「煙燎」、「熟食」諸概念是「禮」的起源。「火」帶來了光明與動能，它是生命的起源，一種精微的生命之火被稱為「精氣」，「精氣」可與「」在本質上可互訓。「」的意識化即是本質性的精神之「本心」，其同格語包含良知、心中之心、大心等；「本心」的自然面即是「」（其同位語包含夜氣、先天之氣、氣母等）。從戰國後即有經營此生命之火的學問，丹道即是修自然面向的「」之學問，儒道的實踐哲學則是由意識下功夫以轉化人的存在狀態，亦即由「」變化氣質，以證超越。「火」由於具備動能義、光明義，它使得人的道德實踐具足了動力因，「勇」、「恥」、「改過」、「不忍」諸種德目皆出自火的精神。在五行當中，唯有「火」行特別彰顯否定性的自我反思、自我批判的力道，它與各大教的「懺悔」、「罪」之意識有銜接之處。

Among the five elements, the origins of the element fire are the most mysterious. Although the other four elements are connected with heaven in one way or another, their essences are nevertheless rooted in earth. By contrast, the source of fire was thought to be in the heavens; in particular, in the sun and the star called “great fire.” The sun, moon and stars all follow set courses, which constituted the origins of the temporal dimensions of the year, month and days. Time is thus the foundation upon which all experience is established, or what Kant calls “form.” Among the sun, moon and stars, the heat emitted by the sun can be viewed as “heavenly fire,” which was used in reverential offerings to the spirits. Fire also made possible cooked food, which gave rise to both civilization and social order. It moreover kindled the concepts of “heavenly fire,” “sacrificial fire,” and “cooked food,” which were the origins of ritual. Fire brought with it illumination and kinetic energy, and as such it was the source of life. One type of subtle life-giving fire was called “refined essence,” and refined essence can fundamentally be read as a synonym of qi. The conscientization of qi was precisely the original mind of the fundamental nature’s refined essence, and similar concepts describing this mind include the “innate knowledge of the good,” the “mind within the mind” and the “great mind.” The original mind’s natural aspect is qi (its appositional phrases include night qi, innate qi, and the mother of qi). Following the Warring States period, individuals began to study this life-giving fire. The dao of interior alchemy is learning dedicated to cultivating the natural aspects of qi; the practical philosophy of the Confucian dao, which seeks to transform the preserved condition of human beings through conscious effort, changes one’s constitution through qi in order to verify transcendence. Due to the fact that fire possesses the meanings of kinetic energy and illumination,it imbued the practical implementation of human ethics with a sufficient motive force. The virtues of courage, shame, correcting errors, and the refusal to countenance wrongs were all derived from the spirit of fire. Among the five elements, only fire manifests the power of negativity inherent in self-reflection and self-criticism. It moreover shares points of connection with the notions of repentance and sin found in the great religions of the world.