经曰∶诸涩枯涸，干劲揭皴，皆属于燥。又曰∶燥胜则干。又曰∶二阳之病发心脾，
有不得隐曲，女子不月，其传为风消，其传为息贲者，死，不治。
《原病式》曰∶涩，物湿则滑泽，干则涩滞，燥湿相反故也。如人身中外涩涩，皆属燥
金之化也。故秋脉涩。涩，燥也；或麻者，亦由涩也。枯，不荣旺也。涸，无水液也。干，
不滋润也。劲，不柔和也。春秋相反，燥湿不同故也。
经曰∶风热火同，阳也；寒燥湿同，阴也。又燥湿小异也。然燥金虽属秋阴而异于寒湿，
故反同其风热也。故火热盛则金衰而风生，缘风能胜湿，热能耗液而反寒。阳实阴虚，则
风热胜于水湿而为燥也。凡人风病，多因热甚，而风燥者，为其兼化以热为主也。然阳实
阴虚，而风热太甚，以胜水湿，金受火制，因而成燥。
燥乃二阳阳明燥金肺与大肠之气之为病也。夫金为阴之主，为水之源，因受火热，则
为燥气之化。寒水生
化之源竭绝于上，而不能下滋肾水，灌溉周身，充荣血脉，则内外燥气之化而为病，皆由此
也。有因火病克伐太过，有因吐利而亡津液，或预防养生误食金石，或房劳致虚，补塞燥剂；
有食味过浓，辛热太多，醇酒炙肉，皆助火而损真阴。日渐煎熬，血液衰耗，使燥热转深而
燥病偏作。在外则皮肤皴揭，在上则咽鼻焦干，在中则水液衰少而烦躁，在下则肠胃枯涩，
津不润而便难，在手足则为痿弱无力，在脉则涩而微，此皆阴血为火热所伤，而肾水失生化
之源也。
刘河间分大便秘结或消渴之类为燥病之里证；身热、皮肤燥涩干疥、爪枯之类为燥病
之表证。而于阳结、阴结、气盛、血少、痰郁、风热可得而悉。
大法∶身表热为燥在表，渴饮水为热在里。
肝气自甚则风胜，加之燥热则病。筋脉劲强紧急而口噤也。
燥热太甚而脾胃干涸，则成消渴。（详见本方。）
风、热、燥并，郁甚于里，故烦满而或秘结。（详见秘结门。）
风、热、燥甚，而筋缓不收，为痿痹。
《黄帝内经》说：凡是出现涩滞、枯槁、干涸、皮肤紧绷皲裂等症状，都归属于燥邪。又说：燥气偏胜则导致干燥。还说：阳明经（胃与大肠）的病变发于心脾，表现为难以启齿的隐痛或情志不畅，女子月经闭止，若病情传变为风消（因风热耗伤阴血导致的消瘦），或传变为息贲（肺积病，呼吸急促），则属死证，无法治愈。
刘完素在《原病式》中说：涩，物体湿润时则滑利，干燥时则涩滞，这是因为燥与湿性质相反的缘故。如人体内外出现的涩滞感，都属于燥金（秋季燥气）的化生作用。所以秋季的脉象呈现涩象。涩，就是燥；或者出现麻木感，也是由涩引起的。枯，是指不荣润旺盛。涸，是指没有水液。干，是指不滋润。劲（僵硬），是指不柔和。春秋季节气候相反，燥湿不同，因此表现各异。
《黄帝内经》说：风、热、火同属阳；寒、燥、湿同属阴。但燥与湿又有细微差别。然而，燥金虽然属于秋季的阴气，却不同于寒湿，反而与风热有相似之处。所以火热盛则金气衰败而风气内生，因为风能胜湿，热能耗伤津液从而反显寒象。阳气实而阴气虚，则风热胜于水湿而化为燥。大凡风病，多因热甚所致，而风燥之病，是因为兼夹了以热为主的化生因素。既然阳气实、阴气虚，且风热太甚，胜过水湿，金受火克制，因而形成燥病。
燥病是二阳（阳明）燥金，即肺与大肠之气失调所致的疾病。金为阴之主，为水之源，如果受到火热侵袭，则化为燥气。寒水生化之源在上焦竭绝，不能向下滋补肾水，灌溉周身，充养血脉，那么内外的燥气化生而为病，都是由此产生的。有的因为火病克伐太过，有的因为呕吐腹泻而亡失津液，或者预防养生时误食金石类药物，或者房劳过度导致虚弱，又服用过于温燥的补剂；有的饮食味道过浓，辛热太多，饮用烈酒、吃烤炙肉类，这些都助长火势而损伤真阴。日渐煎熬，血液衰耗，使燥热转深而燥病发作。表现在外则皮肤皲裂紧绷，表现在上则咽喉鼻腔焦干，表现在中则水液减少而烦躁，表现在下则肠胃枯涩，津液不能滋润而导致大便困难，表现在手足则为痿弱无力，表现在脉象则为涩而微弱，这些都是阴血被火热所伤，而肾水失去生化之源的结果。
刘河间将大便秘结或消渴等类病症归为燥病的里证；将身热、皮肤燥涩干疥、指甲枯槁等类病症归为燥病的表证。而对于阳结、阴结、气盛、血少、痰郁、风热等情况，可以详细辨别。
肝气自盛则风气偏胜，加上燥热则发病。筋脉僵硬强直紧急，导致口噤（牙关紧闭）。
The *Classic of Difficult Issues* (Jing) states: All cases of roughness, withering, drying up, stiffness, and skin cracking belong to dryness. It also states: When dryness prevails, there is dryness. Furthermore, it states: Diseases of the Two Yang (Yangming) originate from the Heart and Spleen, manifesting as difficulties in intimate matters or emotional constraint, and amenorrhea in women. If the disease transmits to become "wind consumption" (emaciation due to wind-heat consuming yin-blood), or transmits to become "breath accumulation" (a lung mass characterized by shortness of breath), it is a death case and is incurable.
Liu Wansu in the *Yuan Bing Shi* (Treatise on the Original Causes of Disease) states: Roughness occurs because objects are smooth when wet and rough when dry; this is due to the opposition between dryness and dampness. Thus, any sensation of roughness inside and outside the human body belongs to the transformation of Dry Metal (the autumnal dry qi). Therefore, the autumn pulse is rough. "Rough" means dry; numbness is also caused by roughness. "Withered" means lacking nourishment and vitality. "Dried up" means absence of fluids. "Dry" means lack of moistening. "Stiff" means lack of flexibility. This is because the climates of spring and autumn are opposite, and dryness and dampness differ.
The *Classic* states: Wind, heat, and fire are Yang; cold, dryness, and dampness are Yin. However, dryness and dampness have slight differences. Although Dry Metal belongs to the Yin of Autumn, it differs from cold and dampness, thus resembling wind and heat. Therefore, when fire-heat is excessive, Metal declines and wind arises, because wind can overcome dampness, and heat consumes fluids, thereby appearing cold. When Yang is excess and Yin is deficiency, wind-heat overcomes water-dampness and transforms into dryness. Most wind diseases are caused by excessive heat, and wind-dryness diseases occur because they involve transformations dominated by heat. Since Yang is excess and Yin is deficiency, and wind-heat is too severe, overcoming water-dampness, and Metal is controlled by fire, dryness is formed.
Dryness disease is caused by the dysfunction of the Qi of Dry Metal of the Two Yang (Yangming), specifically the Lung and Large Intestine. Metal is the master of Yin and the source of Water. If affected by fire-heat, it transforms into dry qi. If the source of Cold Water's generation is exhausted at the upper Jiao, failing to nourish Kidney Water downwards, irrigate the whole body, and enrich the blood vessels, then the internal and external transformations of dry qi causing disease all stem from this. Some cases result from excessive suppression by fire diseases, some from loss of body fluids due to vomiting and diarrhea, some from mistakenly ingesting mineral drugs during preventive health care, some from deficiency due to sexual exhaustion compounded by the use of overly drying tonics, and some from diets that are too rich, excessively pungent and hot, involving strong wine and roasted meats, all of which assist fire and damage True Yin. Day by day, the body is simmered, blood and essence are depleted, making dry-heat deeper and dryness diseases more prevalent. Externally, the skin cracks and tightens; superiorly, the throat and nose become scorched and dry; centrally, fluids decrease leading to irritability; inferiorly, the intestines and stomach become withered and rough, with insufficient fluids causing difficult defecation; in the limbs, there is flaccidity and weakness; in the pulse, it is rough and faint. These are all due to Yin blood being damaged by fire-heat, and Kidney Water losing its source of generation.
Liu Hejian categorized constipation or thirst conditions like Xiao Ke (Wasting and Thirsting) as interior syndromes of dryness; while fever, dry and rough skin, scaly skin, and withered nails were classified as exterior syndromes of dryness. Regarding Yang constipation, Yin constipation, Qi excess, Blood deficiency, Phlegm stagnation, and wind-heat, these can be fully distinguished.
The general principle: Fever on the body surface indicates dryness in the exterior; thirst and desire to drink indicate heat in the interior.
If Liver Qi becomes excessive, wind prevails; combined with dry-heat, disease occurs. The sinews and vessels become stiff, strong, and tense, leading to trismus (lockjaw).
If dry-heat is too severe and the Spleen and Stomach become dried up, Xiao Ke (Wasting and Thirsting) forms. (See detailed discussion in the relevant formula section.)
When wind, heat, and dryness combine and stagnate severely in the interior, it causes irritability, fullness, and possibly constipation. (See detailed discussion in the Constipation section.)
When wind, heat, and dryness are severe, and the sinews become flaccid and unable to contract, it results in Wei Bi (flaccidity and obstruction).