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Yang Qi (Yang Energy), belonging to the Heavenly Qi, primarily governs the exterior. Yin Qi (Yin Energy), belonging to the Earthly Qi, primarily governs the interior. Therefore, Yang is on the outside as the envoy of Yin; Yin is within as the guardian of Yang. Thus, the three Yang meridians in the human body are like the multiple gates on the outside; the three Yin meridians are like the halls and chambers on the inside. When pathogenic factors invade the Yin aspect resulting in a Yin Syndrome (a syndrome characterized by cold, deficiency, and internal stagnation), it is because the outer gates are not secure, fundamentally due to Yang Deficiency (deficiency of Yang energy). Although the outer city wall has been breached, the inner city can still be defended. Since Yang arises from Yin, if the interior Qi is substantial, it can still resist external aggression. However, if the exterior Qi is slightly deficient and the interior Qi fails to hold fast, then pathogenic factors will invade the Yin aspect.

There was once a practitioner who arrived at a patient's home riding in an ornate carriage, dressed in splendid attire, walking with pride. Upon examining the patient's pulse and symptoms, which presented as deficiency and cold, he stated, "This is a Yin Syndrome." The patient replied, "Incorrect. I have been maintaining my health for several months." The patient's wife, standing by and greatly indignant, feigned laughter and mocked him. The practitioner, feeling ashamed and stammering, dared not use warming and tonifying herbs (herbs that warm the body and tonify deficiency) and left hastily, appearing as if he had lost something. How tragic! To practice medicine without understanding the principles of Yin and Yang is like ordering someone to fight without handing them a weapon.

Zhang Zhicong commented: In the principle of Yin and Yang, the density and security of Yang are essential. If Yang Qi is not secure, pathogenic factors will enter sequentially. Sorrow, fear, grief, joy, and anger all damage the Five Zang Organs (Liver, Heart, Spleen, Lung, Kidney), and pathogens subsequently exploit the damaged organs. This is not solely dependent on the retention of Kidney Essence (Jing).

Qibo stated: The onset of all diseases arises from wind, rain, cold, and heat; imbalances in Yin and Yang; excessive emotions such as joy and anger; improper diet and living habits. Sudden great fright or terror causes Blood and Qi to separate, Yin and Yang to disperse, collaterals to become obstructed and severed, pulse pathways to become blocked, Yin and Yang Qi to reverse each other, Wei Qi (Defensive Qi) to stagnate, meridians to become empty, and Blood and Qi to lose their proper sequence, thereby deviating from normalcy. The *Tong Ping Xu Shi Lun* (Treatise on Evaluating Excess and Deficiency) states: "When pathogenic factors are exuberant, it is Excess (Shi); when essence and Qi are depleted, it is Deficiency (Xu)." It also says: "Wherever pathogens gather, Zheng Qi (Upright/Healthy Qi) must be deficient." Thus, all diseases involve injury by pathogenic factors leading to the depletion of Zheng Qi. Symptoms such as withering of the large bones, sinking of the large muscles, fullness in the chest, and difficult breathing are caused by external contraction of Wind-Cold and internal injury from the Five Emotions. Therefore, in treating any disease, one must first dispel the pathogens, regulate Blood and Qi, smooth out reversals, and unblock stagnations. Only then can Yin and Yang achieve harmony, and Zheng Qi naturally recover. If one only knows how to tonify deficiency without clearing the pathological factors, as long as the pathogen remains, Zheng Qi cannot recover. Gradually accumulating over time, this leads to irrecoverable collapse of deficiency. Furthermore, it is common to see young children who, after contracting external pathogens, develop fever, cough, hemoptysis, or night fever. If the pathogens are not cleared, it leads to the dispersion of Yin and Yang and the drying up of Blood and Qi, resulting in death within months or a year. Yet, these cases are termed "Hundred-Day Weakness" or "One-Year Weakness." How tragic! Young and vigorous individuals have increasingly abundant spirit and daily growing Blood and Qi. It is unheard of for them to become weak and deficient without being caused by pathogenic diseases. Cases of weakness not caused by pathogenic diseases belong to "Qi Heng" diseases (abnormal diseases not caused by external contraction or internal injury). The *Da Qi Pian* chapter states: "If the Stomach pulse is deep, beating forcefully yet choppy, or superficially beating forcefully and large; if the Heart pulse is small, tight, and urgent, these indicate Obstruction syndromes and Hemiplegia. In men, it manifests on the left; in women, on the right. Those under twenty years old will die within three years." (Manifesting from the interior to the exterior is called 'Fa'.) The Ying-Wei, Blood, Qi, skin, flesh, sinews, and bones of humans are all nourished by the refined essences of water and grain from the Stomach-Fu organ. A Stomach pulse that is deep, beating forcefully yet choppy indicates Stomach Qi deficiency and declining generative Qi. Because Blood and Qi cannot nourish the body, Hemiplegia results. For those under twenty, whose spirit is flourishing and Blood and Qi are abundant, presenting such signs of decline indicates that the congenital Yuan Qi (Original Qi) was thin, and postnatal nourishment failed to support it, resulting in self-damage. However, even so, it takes three years to reach a fatal state, not causing immediate death. Examining and distinguishing between Pathogens, Zheng Qi, Excess, and Deficiency is the critical juncture in clinical practice. Famous doctors have many patients; if they can differentiate and treat accordingly, there may be hope for recovery.