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In the context of *Shanghan* (Cold Damage), there is a principle that is extremely prominent and crucial, which Zhang Zhongjing did not explicitly state. Later generations groped in the dark, completely failing to grasp it. The only phrase that hints at this is "Shanghan tends to kill those with deficiency in the lower jiao," but this is often interpreted retrospectively after the patient's condition has deteriorated, blaming the patient's own lack of caution, rather than recognizing the errors in treatment methods and medication.

Generally speaking, when one contracts cold in winter, the injury usually occurs in the lower jiao, specifically around the knees and shins. If the injury occurs on the head, face, shoulders, or back, these are pathways where yang qi constantly ascends and disperses outward; consequently, symptoms such as aversion to cold and fever would appear immediately. Whether contracted in the morning and manifesting in the evening, or contracted in the evening and manifesting in the morning, the pathogen typically dissipates upon sweating, and sometimes resolves spontaneously without sweating. How could it possibly penetrate deeply into the interior, leading to transmission through the six meridians and presenting various illusory or complex syndromes?

The reason for such transmission lies precisely in the fact that the pathogen is latent in the lower part, not clashing with yang qi, allowing it to reside peacefully for a long time. It gradually invades the interior, and over time, rises upward, passing the knees and entering the abdomen. This prevents yang qi from settling in its proper residence, thereby triggering the onset of disease. If this is compounded by a new external contraction in the upper jiao, the onset will be even more violent. To treat this, one must select warming and dispersing herbs that possess sinking and descending properties, or add such agents to warming and dispersing formulas to guide the pathogen downward. This allows the pathogen, which originally invaded from below upwards, to retreat back to the lower jiao and exit from there. When the pathogen retreats downward, the primordial qi returns to its root, and the body becomes secure. Herbs with sinking and descending properties include *Weilingxian* (Clematis Root), *Duhuo* (Angelicae Pubescentis Radix), *Wuzhuyu* (Evodiae Fructus), *Chenxiang* (Aquilariae Lignum), *Niuxi* (Achyranthis Bidentatae Radix), and *Zexie* (Alismatis Rhizoma). If one exclusively uses *Mahuang* (Ephedrae Herba) and *Guizhi* (Cinnamomi Ramulus) to induce sweating in the upper jiao, the upper jiao's primordial qi will become increasingly empty after sweating. Consequently, the pathogen in the lower jiao will shift further upward due to this emptiness. This is not expelling the pathogen; it is essentially driving the healthy qi to invite the pathogen inward. Therefore, ancient practitioners considered the pathogen moving from the Taiyang meridian down to the Shaoyin meridian as "deepening," and moving from Taiyang/Shaoyin up to the Yangming/Taiyin meridians as also "deepening."

After inducing sweating, if abdominal distension and fullness occur, along with the inability to retain water or medicine, this indicates not only that damp-heat in the middle jiao has been disturbed by sweat, but also that cold-pathogen from the lower jiao has risen upward and clashed with the dampness in the middle jiao. After sweating, vomiting, or purging, if epigastric fullness with a sensation of rebellion, Qi rushing up to the chest, dizziness, palpitations, and tremors occur, this signifies not merely the disturbance of clear Yang and fluids in the upper jiao meridians by sweat, but the upward surge of lower jiao cold-pathogen causing chaotic reversal of Qi and blood in all vessels, preventing them from following their normal meridional courses. After purging, if rapid pulse, chest fullness, irritability, fright, inability to urinate, delirium, and heaviness of the body preventing turning occur, this results from the clash between the residual medicinal force in the upper stomach and the lower jiao cold-pathogen. Two types of cold clash, suppressing Yang qi in the middle jiao and preventing its free flow. In the *Bian Mai Pian* (Chapter on Pulse Differentiation), it states: if the pulse is floating and large, and the physician erroneously uses purgation, followed by drinking cold water, the combination of water and cold will cause specific adverse reactions, similar to the cases described above. This corresponds to the concept of "Taiyang following the meridian, with stasis-heat residing in the interior," leading to blood accumulation and jaundice. This too is caused by cold-pathogen contesting upward, preventing blood vessels from flowing downward normally. The fundamental reason is that *Mahuang* and *Guizhi* can only induce sweating from the exterior of the upper jiao, but cannot search out and eliminate the latent cold in the lower jiao. Diseases of Shanghan are typically characterized by pathogens latent in the lower jiao, accumulating depression for a long time until triggered. They do not reside in the critical pathways of Qi and blood entry and exit in the upper jiao. Therefore, medicinal power cannot encounter them directly. Instead, it weakens the upright Qi in the upper jiao, paving the way for the pathogen to rise wildly and aggressively. Is it surprising then that the pathogen spreads uncontrollably and changes unpredictably?

One might ask: At the initial stage of Taiyang disease, stiffness and pain in the neck and back—are these not diseases of the upper jiao? At this time, is there no discomfort in the knees and shins? I reply: The stiffness and pain in the neck and back are precisely caused by the lower jiao pathogen rushing upward, supporting itself against the new external contraction. At this moment, the area below both thighs must surely feel abnormal distension and pain. How can one say there is no discomfort? If there were truly no discomfort, it would merely be a common cold, requiring no extensive effort from sages past and present to analyze.

Therefore, when treating Shanghan, one must first examine the deficiency/excess and cold/heat status of the Yuan Qi (Primordial Qi) in the lower jiao. If deficient and cold, it indicates insufficiency of True Yang, known as "lower jiao deficiency"; treatment requires warming, searching out the pathogen, and tonifying. If deficient and hot, it indicates dryness-fire; treatment requires warming, searching out, and moistening/dissolving. If excess and cold, treatment requires strong warming and searching out. If excess and hot, it indicates pathogen in the exterior collaterals with internal stasis-heat; treatment requires warming, searching out, and clearing/dissolving heat. When medicinal power reaches the site of the disease, the pathogen exits via the same route it entered, and any new wind-cold in the upper jiao will naturally dissipate alongside the warming/searching herbs. Furthermore, if a patient has inherent weakness of Spleen and Lung, stubborn dampness in the upper jiao exterior, phlegm-fluid retention in the middle jiao, prolonged heat accumulation in the middle jiao, or deficient fire frequently rising to the head and face, appropriate herbs can be added according to the syndrome. Understanding this principle allows one to comprehensively treat Shanghan complicated by Hong Kong foot (gouty arthritis/jiaojixi), hernia, masses, fluid retention, or heart-stomach Qi pain, without needing excessive manual intervention. Even with blood masses or hemorrhagic conditions, blood-invigorating herbs can be added to the treatment without damaging the upper jiao. One can thus maneuver freely and effectively, avoiding the proliferation of complicated syndromes and difficult-to-handle situations.

I often see patients who contracted cold in winter developing symptoms after the Rain Water or Awakening of Insects solar terms: sudden severe vomiting, hiccups, dyspnea, diarrhea, vertigo with a feeling of emptiness in the chest, weakness in the legs leading to panting after walking only a few steps, sudden obstruction of urination and defecation, or sudden profuse diarrhea, inexplicable irritability, insomnia, upper heat with lower cold, irritability and wheezing, or feet becoming burning hot after midnight. These are accompanied by pain throughout the bones and sinews, especially severe below the thighs. Although the body is febrile, the heat comes and goes irregularly, unlike exterior syndromes. The world mistakenly classifies these as miscellaneous diseases, unaware that they stem from latent winter cold in the lower jiao. If combined with a new upper jiao contraction, reappearing as aversion to cold and fever, and treated solely as an exterior syndrome with *Mahuang* and *Guizhi*, it leads to floating Qi causing wheezing, edema in the legs, and loss of the root of Yuan Qi. Additionally, there are cases where summer-heat was latent internally, and upon entering winter, cold-pathogen surged upward from below, triggering illness. Symptoms include vexing heat in the chest resembling skin rupture, ice-cold feet, and feet becoming burning hot at night. This is due to the clash of cold and heat causing stasis and obstruction in the blood collaterals of the three jiaos. Such conditions are rarely understood by practitioners.