Taibai Zhenren Song
    “Xishan Qunxian Hui Zhen Ji” volume 1 quotes Taibai Zhenren as saying: ‘The Five Elements are inverted. The dragon comes out of the fire; the Five Elements do not flow smoothly. The tiger is born in the water.’ It also explains: ”The dragon is originally a thing of the East, Jia and Yi, but those that come out of the fire are born on the surface of the heart's liquid, the positive yang energy. The tiger is a thing of the West, Geng and Xin, but those that are born in the water are born in the middle of the kidney's energy, the true yin water. The same book, volume 3, quotes the “Mirror of Medicinal Herbs” as saying: “The kidney produces vital energy, and within that energy is hidden the true and unique water, which is called the yin tiger. The heart produces vital fluid, and within that fluid is hidden the positive yang energy, which is called the yang dragon.” According to the ancient theory of yin and yang and the five elements, the green dragon is associated with the east and the element wood, and in neidan it is used to refer to real mercury; the white tiger is associated with the west and the element metal, and in neidan it is used to refer to real lead. Wood gives birth to fire, and metal gives birth to water. This is the universal principle of the five elements. However, for the cultivation of internal alchemy, the heart belongs to the element fire and is paired with the hexagram of separation. The heart gives birth to true liquid, and within the true liquid is hidden the breath of the one, which is the true mercury (symbolized by the green dragon). This is fire giving birth to wood. The kidney belongs to the element water and is paired with the hexagram of坎. The kidney gives birth to true qi, and within the true qi is hidden the water of the one, which is the true lead (symbolized by the white tiger). This is water giving birth to metal.
太白真人歌
    《西山群仙会真记》卷一引太白真人曰：“五行颠倒术，龙从火里出；五行不顺行，虎向水中生。”并解释说：“龙本东方甲乙之物，而出于火中者，心液之上，正阳之气也，则曰阳龙出自离宫。虎乃西方庚辛之物，而生于水中者，肾气之中，真一之水也，则曰阴虎生于坎位。”同书卷三引《入药镜》曰：“肾中生气，气中暗藏真一之水，名曰阴虎。心中生液，液中暗藏正阳之气，名曰阳龙。”按古代阴阳五行学说，青龙配东方，属木，内丹中喻指真汞；白虎配西方，属金，内丹中喻指真铅。木生火，金生水，此为五行之常理。然对内丹修炼来说，心属火，配离卦，心中生真液，真液之中藏真一之气，即真汞（以青龙喻之），此为火生木；肾属水，配坎卦，肾中生真气，真气之中藏真一之水，即真铅（以白虎喻之），此为水生金。