阳者，天气也，主外。阴者，地气也，主内。故阳在外，阴之使也；阴在内，阳之守也。是以人之三
阳，犹外之重门；人之三阴，犹内之堂室。邪中于阴而为阴证者，乃重门不固，本阳虚也，虽然外城已破，
内城尚可固守。盖阳生于阴，里气实者，犹能外御其侮。若表气微虚，里气不守，则使邪中于阴矣。有一
方士，华舆盛服，昂昂然至病者之家，审其脉证虚寒，曰∶此阴证也。病者曰∶不然。予保养数月矣。病者之
妻，亦大愠于侧，佯哂而讥之。方士赧言语涩，遂不敢用温热之药，贸贸然而出，若有所失。悲夫！不
明阴阳之道而为医，犹未能操戈而使之战也。
开之曰∶阴阳之道，阳密为要。如阳气不固，则邪不以次入，忧、恐、悲、喜、怒，皆伤五脏，邪即
随所伤之脏而乘之，又非独肾精之能固守也。
岐伯曰∶夫百病之始生也，皆生于风雨寒暑，阴阳喜怒，饮食居处。大惊卒恐，则血气分离，阴阳破散，经
络厥绝，脉道不通，阴阳相逆，卫气稽留，经脉虚空，血气不次，乃失其常。《通评虚实论》曰∶邪气盛则实，精
气夺则虚。又曰∶邪之所凑，其正必虚。是凡病未有不为邪气所伤，而即为正气虚脱者也。是以大骨枯槁，大
肉陷下，胸中气满，喘息不便，皆因外感风寒，内伤五志之所致。故凡病当先却其邪，调其血气，顺其所逆，通其
所稽，则阴阳和平，而正气自复。若止知补虚，而不清理其病，邪病一日不去，正气一日不复，渐积至久而成不
救之虚脱矣。又常见少年子女，因感外邪，而为发热，咳嗽，或为唾血，或为夜热，不行清理其邪，而致阴阳破
散，血气干枯，有不数月而死者，有不周岁而死者，而曰此百日怯也，此周年怯也。悲夫！夫少壮之人，精神日
盛，血气日生，若不因邪病而成虚怯，未之有也。有不因邪病而成虚怯者，奇恒之病也。（不因外感内伤，故曰奇恒。）《大
奇篇》曰∶胃脉沉鼓涩，胃外鼓大，心脉小紧急，皆隔，偏枯，男子发左，女子发右。年不满二十者，三岁死。（从内
而外，故曰发。）夫人之荣卫血气、皮肉筋骨，皆资生于胃腑
水谷之精。胃脉沉鼓涩者，胃虚而生气衰也。血气不能荣养于身，故成偏枯之证。年未满二十者，精神正
盛，血气方殷，而反见此衰败之证，此因先天所秉之元气虚薄，而后天不能资培，斯成自损之病，然亦至三
年之久，而不致于速死。审辩邪正虚实，临证要紧关头，名医之门多疾，若能分别救治，庶几其有瘳乎！
阳气，属于天气，主要主管体表；阴气，属于地气，主要主管体内。所以阳气在外，是阴气的护卫；阴气在内，是阳气的守备。因此，人体的三阳经，就像外面的重重门户；人体的三阴经，就像里面的厅堂居室。邪气侵入阴分而成为阴证的情况，是因为外部的门户不牢固，根本原因在于阳气虚弱。虽然外层的城墙已经被攻破，但内层的城池还可以坚守。因为阳气生于阴中，如果里气充实，仍然能够抵御外来的侵犯。如果表气轻微虚弱，里气不能固守，那么就会使得邪气侵入阴分。
曾有一位方士，乘坐华丽的车舆，穿着盛装，昂首挺胸地来到病人家中。他诊察病人的脉象和症状，发现呈现虚寒之象，便说：“这是阴证。”病人却说：“不对。我已经保养了几个月了。”病人的妻子也在旁边非常生气，假装嘲笑并讥讽他。方士感到羞愧，说话结巴，于是就不敢使用温热的药物，糊里糊涂地离开了，好像失去了什么一样。可悲啊！如果不明白阴阳的道理就去做医生，就像还没有学会拿武器却让人去打仗一样。
张志聪评论说：阴阳的道理中，阳气致密是关键。如果阳气不固，邪气就会依次侵入，忧、恐、悲、喜、怒这五种情志都会损伤五脏，邪气随即乘虚进入所伤的脏腑，这不仅仅是肾精能够固守的问题。
岐伯说：各种疾病的发生，都源于风雨寒暑等外感因素，以及阴阳失调、喜怒无常、饮食起居不当。突然的大惊或大恐，会导致血气分离，阴阳破散，经络阻滞断绝，脉道不通，阴阳之气逆乱，卫气停滞，经脉空虚，血气运行失去次序，从而违背了正常的生理规律。《通评虚实论》说：邪气亢盛就是实证，精气耗损就是虚证。又说：邪气聚集的地方，正气必然虚弱。所以凡是疾病，没有不是被邪气所伤，进而导致正气虚脱的。像大骨枯槁、大肉陷下、胸中气满、呼吸喘息不便等症状，都是由于外感风寒、内伤情志所致。所以凡是有病，应当先祛除邪气，调理血气，顺应其逆乱之势，疏通其停滞之处，这样阴阳才能平和，正气自然恢复。如果只知道补虚，而不清理病邪，邪气一天不去，正气就无法恢复，逐渐积累久而久之，就会形成无法挽救的虚脱。又常见少年子女，因为感受外邪而出现发热、咳嗽，或者咳血，或者夜间发热，如果不及时清理邪气，就会导致阴阳破散，血气干枯，有的不到几个月就死亡，有的不到一年就死亡，却称之为“百日怯”、“周年怯”。可悲啊！年轻力壮的人，精神日益旺盛，血气日渐充盈，如果不因邪病而变成虚怯，是从来没有的事。有不因邪病而变成虚怯的，属于“奇恒”之病（不因外感内伤，所以称为奇恒）。《大奇篇》说：胃脉沉取鼓指且涩，胃脉浮取鼓指且大，心脉小且紧急，这些都是隔证、偏枯证，男子发于左侧，女子发于右侧。年龄不满二十岁的，三年后死亡。（病从内部发展到外部，所以称为发。）人的营卫血气、皮肉筋骨，都依赖于胃腑水谷的精微物质来滋养。胃脉沉取鼓指且涩，说明胃气虚弱，生气衰少。血气不能荣养身体，所以形成偏枯的病症。年龄未满二十岁的人，精神正旺盛，血气正充沛，反而出现这种衰败的病症，这是因为先天禀受的元气虚薄，而后天又不能资助培养，这才形成自我损耗的疾病，但也经过三年之久，不至于迅速死亡。审辨邪正虚实，是临证时的关键关头，名医门下多病患，如果能分别救治，或许就有痊愈的希望吧！
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.