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The human body is originally formed by the transformation of Yin and Yang Qi. When these two energies are balanced and harmonious, a person remains free from disease; however, if one side becomes excessive, illness arises. Since ancient times to the present day, although there have been numerous treatment methods, they all essentially boil down to correcting imbalance and remedying defects. For deficiency patterns, tonification is used; for excess patterns, purgation is employed. By rectifying the excessive tendency and restoring harmony, the disease will be cured.

Yin and Yang are the source of heaven, earth, and all things. The Six Excesses (Liu Yin) of nature, the Seven Emotions (Qi Qing) of humans, and the properties of medicinal herbs all derive from this principle.

Regarding the human body: Qi belongs to Yang, Blood belongs to Yin. Wei Qi (Defensive Qi), which circulates on the exterior, is Yang; Ying Qi (Nutritive Qi), which nourishes the interior, is Yin. The Six Fu Organs belong to Yang, while the Five Zang Organs belong to Yin. The upper half of the body belongs to Yang, and the lower half belongs to Yin. The innate Yin and Yang reside in the Kidney and Mingmen (Gate of Vitality); the acquired Yin and Yang reside in the Spleen and Stomach. The reasons why humans can fill their bodies, moisten their hair, steam and transform food and drink, and warm and nourish circulation are all due to the functional expression of Yang Qi. However, Yang Qi cannot exist in isolation; it must rely on Yin Blood to moisten it. The formation of form and substance, as well as maintaining moisture and flow, are all sustained by Yin Blood. Therefore, although the Zang-Fu organs and limbs have differences in Yin and Yang, there is no place in the internal and external body that does not possess Yin and Yang Qi.

When Yin and Yang combine, life exists; when they become imbalanced, disease occurs; when they separate, death ensues. In the onset of disease, significant imbalance leads to severe illness, while slight imbalance leads to mild illness. Human death results either from the exhaustion of Yang or the depletion of Yin.

Furthermore, individuals are born with different constitutional predispositions. For example, those with dark or reddish skin, large bones, and thin builds are considered Yang constitutions; those with soft white skin, small bones, and fat builds are considered Yin constitutions. In diseases affecting obese individuals, one must fear the deficiency of Yang; in diseases affecting thin individuals, one must worry about the drying up of Yin. The Six Excesses of nature also invade based on the body's weakness: individuals with Yin deficiency are prone to contracting Wind, Dryness, Summer-heat, and Fire pathogens; individuals with Yang deficiency are prone to contracting Cold, Dampness, Fog, and Dew pathogens. Yang tends to transform into Fire, and Yin tends to transform into Water; Water is cold, and Fire is hot. The *Nei Jing* (Inner Canon) states: "Yin deficiency generates internal heat, Yang deficiency generates external cold." Yang excess often manifests as excess patterns, while Yin excess often manifests as deficiency patterns. Understanding Yin and Yang makes the diseases of Exterior/Interior, Deficiency/Excess, and Cold/Heat clear at a glance.

Some say that extreme anger damages Yin, extreme joy damages Yang, overthinking causes Spleen Yang to knot, fear causes Kidney Yang to dissipate, physical labor with sweating causes Wei Qi (Defensive Qi) to loosen, and intense mental strain disturbs Heart Yin. Furthermore, improper use of purgatives damages Yin, improper sweating damages Yang, severe vomiting damages Yang, blood loss damages Yin, acrid-hot substances damage Yin, and bitter-cold substances impair Yang. Extending from this to the Seven Emotions and Six Qi, nothing is not governed by Yin and Yang. Clinicians should only examine the principles of Yin-Yang fullness/emptiness and waxing/waning. Although symptoms may change unpredictably, they do not exceed the harm caused by Yin-Yang deficiency. Treatment should employ methods to correct imbalance and remedy defects. One must not simply establish theories based on Yin or Yang deficiency and rigidly apply formulas like Liuwei Dihuang Wan (Six-Ingredient Rehmannia Pill) or Shenqi Wan (Kidney Qi Pill) as fixed rules. The key lies in clearly observing the cause of the disease before treating; doing so achieves more than half the success.

For instance, Shanghan (Cold Damage) involves injury to Exterior Yang, treated with Acrid-Warm herbs to disperse Exterior Cold. If it is Wenre (Warm Heat), it involves blazing Interior Yin-Heat, treated with Bitter-Cold herbs to overcome Interior Heat. Extending this logic to Yang Edema and Yin Edema, Yang Jaundice and Yin Jaundice, Yang Collapse and Yin Collapse, Yang Summer-heat and Yin Summer-heat, Yang Malaria and Yin Malaria, Yang Mania and Yin Epilepsy, Yang Carbuncle and Yin Ulcer, all fall under the path of Yin-Yang excess.

Let us first discuss diseases caused by Yang pathogens: such as rapid pulse, fever, constipation, dry orifices, irritability, yellow-black tongue coating, thirst, and desire for drinking. Diseases caused by Yin pathogens present with slow or tight pulses, white slippery/greasy tongue coating, pale/clear complexion, moist orifices, loose stools, and clear urine.

If Yin pathogens are determined to be in the Exterior, methods using Mahuang Tang (Ephedra Decoction) or Guizhi Tang (Cinnamon Twig Decoction) are applicable. If Yin pathogens are known to be in the Interior, methods using Sini Tang (Frigid Extremities Decoction) or Lizhong Tang (Regulate the Middle Decoction) are applicable. To treat Yin Excess patterns, formulas such as Sanwu Baisan (Three-Ingredient White Powder) or Fuzi Xie Xin Tang (Aconite Draining the Heart Decoction) are used. To treat Yang Excess patterns, formulas such as Baihu Tang (White Tiger Decoction) or Huanglian Tang (Coptis Decoction) are used. In severe cases, methods involving Chengqi Tang (Purge the Ganglion Decoction) or Xiongxiong Tang (Drain the Chest Decoction) are employed, or agents to build the middle and support Yang Qi, and formulas like Maishengmai San (Pulse Reviving Powder) to rescue Yin fluids.

There are also cases of Yin Excess, where externally there is aversion to cold and cold limbs, and internally turbid Yin rises upward. Invading Clear Yang, this causes headache, throat obstruction, vomiting, wheezing/cough, hiccups, cholera-like disorder, chest bi-syndrome, phlegm-fluid retention, edema, and diarrhea. Cold coagulation and blockage cause chest, hypochondriac, and abdominal pain. When Yin Excess reaches its extreme, one may see hallucinations ("seeing ghosts"), experience restlessness, sweat profusely leading to collapse, and die.

If Yang is hyperactive, externally there is body heat and bone-steaming fever; internally, Fire Qi flares upward. Scorching the clear pathways, it also causes headache, swollen throat, nausea/vomiting, Xiao Ke (Wasting-Thirst), wheezing/cough, cholera-like disorder, phlegm knots, urgent diarrhea, macules/jaundice, and madness. Dryness and accumulation causing blockage also lead to chest, hypochondriac, and abdominal pain. In severe cases, delirium, blindness, coma, and cessation of breath occur.

There are also cases where Extreme Yang resembles Yin, and Extreme Yin resembles Yang, which is most confusing. For example, externally one may have a red face, irritability, and dislike wearing clothes, but the pulse is weak upon deep pressure. Although thirsty, one does not drink much. The tongue coating is yellow but slippery. Urine and stool are not yellow-red nor dry/hard. In severe cases of intense Interior Heat, Yin may be repelled to the exterior, causing a sensation of cold limbs, aversion to cold, and shivering. This is "deeper heat leads to deeper cold extremities" (Re Shen Jue Shen). The pulse feels deep, rapid, and forceful. The mouth is dry and thirsty with desire to drink, the tongue is dry and lusterless, the coating is often yellow-black with cracks, and urine/stool are yellow-red and constipated/difficult. The key is to differentiate Deficiency and Excess to prescribe medication, thereby avoiding confusion from other divergent paths.

In summary, precise and detailed syndrome differentiation, and seeking the root through pulse diagnosis, allow one to hold simplicity to manage complexity, making grasping the essentials easier. Unlike modern medical books that rigidly adhere to disease names for treatment—leading to feelings of being overwhelmed and scattered, difficult to verify, thus failing to see the disease's source and instead breeding doubt and error—I have now summarized them concisely, clearly, and explicitly, so that future learners have a clear direction.

Commentary by Lan: Exterior/Interior, Deficiency/Excess, Root/Branch, Yin/Yang. Understanding these eight characters, although myriad diseases may transform endlessly, using them for deduction offers the wonder of "different routes leading to the same destination." The Classics state: "He who knows the essentials concludes in one word; he who does not know the essentials wanders infinitely." From this, we see that medicine values broad knowledge of past and present, surpassing former sages. Only those with solid academic foundations can speak of this. The arguments presented by Mr. here are methods of shaking dust from clothes and lifting the collar (getting to the essentials). They cannot be mentioned in the same breath as modern medical books that stick to fixed prescriptions for treatment. If one can devote oneself to careful observation, read thoroughly, and think deeply, one will naturally acquire the skill of handling matters with ease. With confidence in mind, one will avoid echoing others and mistakenly harming people by rigidly adhering to warming and tonifying methods.