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Fuling (Poria) forms in the soil and remains unchanged for a long time, which indicates that it absorbs a significant amount of Yin Qi (Yin Energy), comparable to Zhuling (Polyporus). However, the maple tree has the ability to attract rain, while the pine tree stands tall without withering; one leans towards Yin, and the other towards Yang. Substances leaning towards Yang can consume Yin but cannot generate it; if they cannot generate Yin, they cannot relieve thirst. Therefore, Wuling San (Five-Ingredient Powder with Poria) is used for thirst following sweating; if there is no thirst, Fuling Gancao Tang (Poria and Licorice Decoction) is primarily used. Gualou Qumai Tang (Trichosanthes Root and Rhizoma Phlomi Decoction) treats thirst, where Fuling is present but Gualou (Trichosanthes Root) is indispensable. In Xiaochaihu Tang (Minor Bupleurum Decoction), when thirst occurs, Renshen (Ginseng) is added; in Xiaoqinglong Tang (Minor Green Dragon Decoction), when thirst occurs, Gualou is added. In neither case is Fuling added alone, which demonstrates that Fuling is not a herb for treating thirst. The herbs capable of generating Yin to relieve thirst are none other than Gegen (Kudzu Root) and Gualou. Since Gegen and Gualou generate Yin without promoting urination, their efficacy in relieving thirst is preserved. Herbs that generate Yin while also promoting urination, such as Zhuling and Zexie (Alisma), have reduced efficacy in relieving thirst, placing them after Gegen and Gualou, with Fuling ranking even lower. However, Wuling San and Zhuling Tang (Polyporus Decoction) specifically use these herbs to treat thirst. Why is this not replaceable by Gegen and Gualou? It is because this thirst is not due to other causes, but rather presents with floating pulse, fever, desire to drink water, and difficulty in urination. If the pathogenic factor is not removed, how can generating Yin help? When Fuling, combined with Zhuling and Zexie, drains water, urination becomes smooth. When Fuling and Zhuling, combined with Guizhi (Cinnamon Twig) and Huashi (Talc), release exterior pathogens, the exterior syndrome resolves. By removing the factors that obstruct and scorch Yin, Yin naturally rises. As Yin rises, thirst stops. This is why Fuling earns its place among herbs for treating thirst.

Nevertheless, there are crucial distinctions that must be clarified. The use of Er Ling (Fuling and Zhuling) and Zexie to treat thirst addresses thirst caused by excessive water intake coupled with impaired urination. Because this stagnant water cannot be transformed by the Stomach into fluids, it leads to vomiting; if it blocks Kidney Yin from ascending, it causes thirst. Thus, draining water and promoting urination cures it. However, if it is Phlegm-Fluids (Tan-Yin), the Stomach relies on it for nourishment; thick Phlegm-Fluids do not naturally exit via urination. Does this mean water causes thirst, but drinking fluids does not?! Yet Zhang Zhongjing stated: "Those who frequently vomit should normally feel thirsty; if they do not feel thirsty, it is due to Zhi Yin (branching fluid retention) under the heart." He also said: "When Liu Yin (retained fluid) is in the chest, the person suffers from shortness of breath and thirst." These two statements seem contradictory. What is the reason? "Zhi" implies branching or deviating flow. If the flow is not direct, Kidney Yin can still ascend to moisten, so there is no thirst. "Liu Yin" refers to fluid retained directly in the chest; thus, Qi movement is hindered (shortness of breath) and thirst arises. This distinguishes Tan (Phlegm) from Yin (Fluids). Water and Yin have both distinct and overlapping aspects. One can decide whether to use Fuling or Zhuling/Zexie based on the presence or absence of thirst.

Furthermore, considering Zhang Zhongjing's use of these three herbs (Fuling, Zhuling, Zexie), the syndromes often involve both thirst and vomiting. Does this imply they treat both? It is unknown whether Banxia (Pinellia) and Shengjiang (Fresh Ginger) are specific remedies for vomiting, just as Gegen and Gualou are specific for Xiao Ke (Wasting-Thirst). In Fugan Wuwei Jiangxin Tang (Poria, Licorice, Schisandra, Ginger, and Asarum Decoction) for cough and fullness, Zhang adds Banxia if vomiting is present. Since Fuling is already included, adding Banxia suggests Fuling does not treat vomiting; similarly, omitting Zhuling and Zexie suggests they do not treat vomiting either. However, in the chapter on Vomiting, Zhuling San (Polyporus Powder) clearly treats vomiting with desire for water, and Fuling Zexie Tang (Poria and Alisma Decoction) clearly treats gastric reversal vomiting with desire to drink water. To claim they do not treat vomiting would be unbelievable. But if they were equivalent to Xiaobanxia Tang (Minor Pinellia Decoction) for vomiting, why are there cases emphasizing desire for and drinking of water? The principle of draining water to stop thirst is clear. If draining water stopped vomiting, the vomiting would cease once the water was gone, without needing drugs. Thus, Zhang's words are profound. The phrase "desire for water" in Zhuling San refers to the "post-vomiting desire for water" mentioned earlier. Here, "desire for water" precedes vomiting. "Thirst followed by vomiting indicates water retention under the heart." When water retains under the heart, increased thirst leads to increased drinking. Gastric reversal involves Stomach deficiency and cold, which typically does not cause thirst. Current thirst with desire to drink indicates a syndrome of Yang within Yin. Hence, the conjunction "er" (and/then) is added to highlight this distinction from other gastric reversals or vomiting types. Jiang (Ginger), Rougui (Cinnamon Bark), Gancao (Licorice), and Baizhu (Atractylodes Macrocephala) warm the Stomach and stop vomiting. Fuling and Zexie drain water and stop thirst. Since symptoms overlap, treatments are differentiated. The presence of Shengjiang without Banxia is because Banxia is contraindicated in thirst. The absence of Zhuling indicates no exterior syndrome. Zexie is superior for draining water and stopping thirst, hence its inclusion. Fuling assists Zexie in draining water, Shengjiang in calming rebellion, Guizhi in transforming Qi, and Gancao/Baizhu in tonifying the Middle. Its benefits are manifold, justifying its position as the chief herb. Compared to Zhuling and Zexie, although none treat vomiting directly, Fuling offers supportive assistance. Why? Sweet flavor enters the Spleen first; bland flavor nourishes the Stomach. Fuling is sweet and bland, unlike Zhuling and Zexie. Thus, regarding vomiting, it uses a soothing approach rather than an aggressive one.

Beyond this, Fuling achieves results through draining water in three other contexts in Zhang's formulas: Xuan (Vertigo/Dizziness), Ji (Palpitations), and Ke (Cough). Differentiating similar symptoms reveals truths consistent with vomiting and thirst. Vertigo can result from Lung Wei (Flaccidity) causing upper deficiency, loss of Essence causing lower depletion, or Gu Dan (Grain Jaundice) due to diet; Fuling should not be applied indiscriminately. However, if Zhi Yin is under the heart causing severe Mao Xuan (dizziness with a sensation of being covered/muffled), Fuling is applicable. Note that Zexie Tang (Alisma Decoction) uses Zexie for Mao Xuan even without thirst, as Zexie specializes in this symptom. Adding Fuling might counteract Zexie's upward action, compromising efficacy. Baizhu dispels fluids and stabilizes the Middle, supporting Zexie effectively, so Baizhu is preferred over Fuling. What distinguishes Mao from Fei Mao? "Mao" implies Upper Yang blocked by fluids from entering Lower Yin, causing Yang to float upwards like a cover, representing excess Yang. Zexie drains water and supplements Yin to resolve this. If fluids overwhelm and Upper Yang cannot contend with Yin, Yin and fluids combine to cause vertigo, representing excess Yin. Fuling, possessing harmonious Yang nature and excelling in transforming Qi, causes water to retreat and Yang to expand. Zhang has many such formulas for vertigo.

Water retention under the heart causing vertigo also causes Ji (palpitations). Vertigo is external, Ji is internal; though different manifestations, they share the same source. Stabilizing vertigo stabilizes Ji. Heart Ji is caused by water invading the Heart; Umbilical Ji originates from the Kidneys. Thus, Fuling may not be the sole remedy. However, water from all three Jiao should exit via urination, making Fuling essential. Initially, Umbilical Ji may occur, later leading to Heart Ji ("Water in Kidney causes Heart Ji"). Palpitations not treated by Fuling include those managed by Xiaojianzhong Tang (Minor Construct the Middle Decoction), Guizhi Gancao Tang (Cinnamon Twig and Licorice Decoction), and Zhigancao Tang (Baked Licorice Decoction), which require warming the Middle and tonifying Heart Yang. Fuling's diuretic nature might harm healthy Qi, so it is excluded.

Causes of Ke (cough) are numerous; Fuling manages coughs due to Phlegm-Fluids. However, some Phlegm-Fluid cases are unsuitable for Fuling: Banxia Mahuang Tang (Pinellia and Ephedra Decoction) treats Phlegm-Fluid with palpitations by using Mahuang (Ephedra) to vent Heart Yang via the exterior; Xu Zhongke calls this "Old Phlegm," which cannot be resolved by leaching. Gansui Banxia Tang (Kansui and Pinellia Decoction) treats retained fluid with purging, using Gansui (Kansui), Gancao, and Shaoyao (Peony) to purge downwards; forcing it via urine would be erroneous. Other Phlegm-Fluid cases requiring acrid dispersion or bitter descent are excluded. Cough is a Lung disease; Fuling's downward leaching fails to resolve Lung pathogens, so it is rarely used for cough. It is effective only when Chong Qi (Rebellious Qi) carries Phlegm-Fluids upward causing chest fullness. Then, Fuling's downward drainage works swiftly. Thus, Fuling treats Phlegm-Fluids, not cough per se; it treats Fluids, not Phlegm. Linggui Zhugan Tang (Poria, Cinnamon Twig, Atractylodes, and Licorice Decoction) treats Phlegm-Fluids miraculously, with Fuling as the sovereign herb to guide subtle fluids out via urine, highlighting the necessity of Fuling for Phlegm-Fluids.

Its ability to drain water/fluids from all three Jiao via urination stems from its properties. Fuling is sweet and bland, a correct herb for the Stomach. Its pure white color associates it with the Lung. The Lung governs skin/hair and corresponds to the Taiyang (Greater Yang) channel. Thus, it enters Taiyang. Its bland nature allows entry from skin/hair into Taiyang Fu organs. Lung and Stomach govern descending; Bladder Qi transformation expels urine. This gives its diuretic effect a momentum like water flowing from a high roof. Zhang adds Fuling for difficult urination for this reason.

Promoting urination is Zhang's explicit instruction and a classic tradition. It should not be confused with curing Xiao Ke (Wasting-Thirst). Does promoting urination exhaust its utility? No. Reviewing Zhang's formulas: Shenqi Wan (Kidney Qi Pill) treats difficult urination, Xiao Ke, and frequent urination. Difficult urination indicates Kidney Yin obstruction; Fuling with Guizhi/Fuzi unblocks it. Frequent urination indicates Kidney Yang weakness; Fuling with Guizhi/Fuzi strengthens it. It is also used for exterior dampness (Fangji Fuling Tang), throat obstruction (Banxia Houpo Tang), Chest Bi (Fuling Xingren Gancao Tang), abdominal masses (Guizhi Fuling Wan), tonics (Shuyu Wan), and wind-dispelling formulas (Houji Heisan). Only Fuling, with its sweet-bland taste and mild nature, transforms water within Qi and Qi within Water, adapting to various complex roles while adhering to this principle. Without proper preparation, its true efficacy is lost, and harm may occur.