三因者，内因、外因、不内外因，此说从于先古。其词意尚有发而未尽者，内
因者，皆起于七情，蕴结于内，又兼浓味膏粱，熏蒸脏腑，房欲劳伤，亏损元气，乃
五脏受之，其病由此内发者，但发之多在富贵人及肥胖者，十有八九。其见证疮多
坚硬，根蒂深固，二便不调，饮食少进，外软内坚，平陷无脓，表实里虚，毒多难出，
得此者，即病证之内伤也，故曰内因。外因者，皆起于六淫，体虚之人，夏秋露卧，
当风取凉，坐眠湿地，以致风寒湿气袭于经络；又有房事后得之，其寒毒乘虚深入
骨髓，与气血相凝者，尤重；或外感风寒，发散未尽，遂成肿痛，此肌肉、血脉、筋
骨受之；其病由此外来者，发之多在不善调摄，浇薄劳碌人，十有八九。见证多寒
热交作，筋骨疼痛，步履艰辛，湿痰流毒，以及诸风瘫痪，口眼歪斜，半身不遂，风
湿风温，天行时毒等证得此者，即疾病之外感也，故曰外因。又有不内外因，内无
七情干内，外无六淫伤外，何由来也？其病得之于饥饱劳役，喜怒不常，饮食者，冷
热不调，动作者，勤劳不惜，以致脏腑不和，营卫不顺，脾胃受伤，经络凝滞，故
为疾者，外无六经形证，内无便溺阻隔，其病多生于膜外、肉里、肌肤之间，如瘰 、
痰注、气痞、瘿瘤之属，治法不必发表攻里，只当养气血，调经脉，健脾和中，行
痰开郁治之，法为最善。此是三因，理之尽矣。
三因，指的是内因、外因和不内外因，这一说法源自古代。其含义中还有未尽阐发之处。
所谓内因，都是起因于七情（喜、怒、忧、思、悲、恐、惊），郁结于体内；又兼食厚味膏粱（油腻精细的食物），熏蒸脏腑；加上房事过度导致劳损，亏损元气。这些主要损伤五脏，疾病由此从内部发生。这类病症多见于富贵人家及肥胖之人，占十之八九。其临床表现疮疡多坚硬，根基深固，大小便不调，饮食减少，外表看似柔软而内部坚实，平坦下陷且无脓液，表现为表实里虚，毒邪难以排出。得到这种病证的，属于内伤类的病证，所以称为内因。
所谓外因，都是起因于六淫（风、寒、暑、湿、燥、火六种外感病邪）。体质虚弱的人，在夏秋季节露天睡觉，迎风取凉，或坐卧湿地，导致风寒湿气侵袭经络；又有在房事后感受此邪的，寒毒乘虚深入骨髓，与气血凝结在一起，病情尤为严重；或者是外感风寒，发散不彻底，从而形成肿痛。这些主要损伤肌肉、血脉、筋骨。疾病由此从外部侵入的，多发于不善调养、生活浇薄劳碌之人，占十之八九。其临床表现多为寒热交替发作，筋骨疼痛，行走艰难，湿痰流注毒邪，以及各类风证瘫痪、口眼歪斜、半身不遂，风湿、风温、天行时毒等证候。得到这种病证的，属于外感类的疾病，所以称为外因。
还有一种是不内外因，内部没有七情干扰，外部没有六淫伤害，病从何而来呢？这类疾病得之于饥饱失常、过度劳役，喜怒无常，饮食冷热不调，劳作不知爱惜体力，导致脏腑不和，营卫（营气和卫气）不顺，脾胃受伤，经络凝滞。因此患病者，外部没有六经（太阳、阳明、少阳、太阴、少阴、厥阴）的形证，内部没有大小便的阻隔。这类病多生于膜外、肉理、肌肤之间，如瘰疬（颈部淋巴结结核）、痰注（痰核流注）、气痞（气机阻滞形成的痞块）、瘿瘤（甲状腺肿大等肿瘤）之类。治疗方法不必使用发表（解表）或攻里（泻下）的方法，只应当补养气血，调理经脉，健脾和中，行痰开郁进行治疗，这种方法最为妥善。这就是三因，其中的道理已经阐述详尽了。
The "Three Causes" refer to internal causes, external causes, and non-internal/external causes. This theory originates from ancient times. Its meaning contains aspects that have not been fully expounded.
Internal causes arise primarily from the Seven Emotions (joy, anger, worry, thought, sadness, fear, fright), which become constrained within the body. They are further compounded by a diet rich in rich flavors and fatty foods (Gao Liang), which steam and damage the Zang-Fu organs. Additionally, excessive sexual activity leads to physical exhaustion and depletion of Yuan Qi (Original Qi). These factors primarily affect the Five Zang Organs, causing diseases to originate internally. Such conditions occur mostly among wealthy individuals and those who are obese, accounting for eight or nine out of ten cases. The clinical manifestations include sores that are hard, with deep and firm roots; irregular bowel movements and urination; reduced appetite; an appearance of softness on the exterior but hardness on the interior; flat depressions without pus; a pattern of excess at the exterior and deficiency at the interior; and difficulty in expelling toxins. Patients presenting these symptoms suffer from internal injury-type disorders, hence termed "internal causes."
External causes arise primarily from the Six Excesses (Wind, Cold, Summerheat, Dampness, Dryness, Fire). In individuals with weak constitutions, sleeping outdoors in summer and autumn, cooling off in the wind, or sitting/sleeping on damp ground allows Wind-Cold-Dampness pathogens to invade the meridians. Another scenario involves contracting pathogens after sexual intercourse, where Cold-Toxins penetrate deeply into the bone marrow due to deficiency, coagulating with Qi and Blood, resulting in particularly severe conditions. Alternatively, incomplete dispersion of externally contracted Wind-Cold may lead to swelling and pain. These factors primarily affect muscles, blood vessels, and sinews/bones. Diseases originating from external invasion occur mostly among those who fail to regulate their lifestyle, leading a precarious and laborious life, also accounting for eight or nine out of ten cases. Clinical manifestations often include alternating chills and fever, pain in bones and sinews, difficulty walking, flow of Damp-Phlegm and toxic pathogens, various paralysis syndromes caused by Wind, deviation of the mouth and eyes, hemiplegia, Wind-Dampness, Wind-Warmth, and epidemic toxic diseases. Patients presenting these symptoms suffer from exogenous diseases, hence termed "external causes."
There is also the category of non-internal/external causes. If there are no internal disturbances from the Seven Emotions and no external injuries from the Six Excesses, where do the diseases come from? These illnesses result from irregular eating and drinking, overwork, frequent mood swings, improper dietary temperature regulation, and reckless physical exertion. This leads to disharmony of the Zang-Fu organs, disruption of Ying-Qi and Wei-Qi (Nutritive and Defensive Qi), injury to the Spleen and Stomach, and stagnation of the meridians. Consequently, patients present with no characteristic signs of the Six Meridians (Taiyang, Yangming, Shaoyang, Taiyin, Shaoyin, Jueyin) externally, nor any obstruction of urination or defecation internally. These diseases typically arise between the fascia, muscle layers, and skin, such as Scrofula (Luoli), Phlegm Nodules (Tanzhu), Qi Distension (Qipi), and Goiter/Tumors (Yingliu). The treatment method does not require releasing the exterior (Biao) or purging the interior (Li); rather, it should focus on nourishing Qi and Blood, regulating the meridians, strengthening the Spleen and harmonizing the Middle Jiao, moving Phlegm, and resolving depression. This approach is considered the most appropriate. Thus, the principles of the Three Causes are thoroughly explained.