External causes
Six excesses: wind cold heat damp dry fire
The Western question is "what disease do you h...
The Western question is "what disease do you have"; the Chinese question is "why does your body feel unwell." The doctrine of causes and mechanisms answers the second: what factors tip the body off balance (causes), and how that imbalance develops into symptoms (mechanisms). Chinese medicine sorts causes into three classes: external — the "six excesses" of wind, cold, summer-heat, damp, dryness and fire, six ways climate tips to extremes; internal — the "seven emotions," excesses of joy, anger, worry, pensiveness, grief, fear and fright; and the neither-internal-nor-external — diet, overwork and restlessness. The brilliance of the scheme: it does not quibble over pathogen names, it portrays "where, along the body's relation with its environment, the balance has tipped." For overseas readers this is the most easily shared part of Chinese medicine — catching a cold after getting soaked, a stomachache under stress, "internal heat" after a night without sleep: bodily experience common to all humanity.
Wind, cold, summer-heat, damp, dryness and fire are six normal climates — the "six qi"; when they grow excessive, deficient or out of season (winter warm as summer), they become the pathogenic "six excesses." Each carries a vivid criminal signature: wind is swift and wandering, symptoms that travel (the chief of the hundred illnesses, usually the carrier that smuggles the others in); cold constricts and congeals (after a chill: tight muscles, aching joints, clear-runny nose); damp is heavy, turbid and sticky (heaviness of head and body, a thick greasy tongue coat, lingering courses — the mugginess of the rainy season is its crime scene); dryness parches and wastes fluids; fire (heat) blazes upward and stirs the blood.
The doctrine's essence is systematizing environmental influence on the body — two millennia before the microscope named the pathogens, Chinese medicine explained disease by "climate modeling," and built matching strategies: for wind-cold, acrid-warm dispersal (ginger soup, scallion white); for damp-heat, clearing and draining damp (coix seed, adzuki bean). Modern readers may read the six excesses as a "spectrum of environmental stressors": sudden temperature swings, humidity and barometric pressure are indeed correlated in much research with joint pain, cardiovascular events and respiratory infection — the ancients' climate modeling and modern environmental medicine point the same way.
Chinese medicine wrote emotion directly into etiology: anger injures the liver, excessive joy the heart, pensiveness the spleen, grief the lung, fear the kidney — a sudden rage can fell a person (yang rising), long brooding wastes the appetite (pensiveness knots the qi), fright can cause incontinence (fear drops the qi). This is not poetic rhetoric but distilled clinical observation: emotion moves the qi-machinery, and through the qi reaches the organs — anger drives qi upward (red face, throbbing head), pensiveness knots it (appetite gone), grief exhausts it (listlessness, low voice), fear sends it down (weak legs).
For the modern reader, this doctrine needs almost no translation — chronic stress is known to affect digestion, immunity, the cardiovascular system, endocrine function; "mind-body medicine" and the "gut-brain axis" are the modern chapters of the seven-emotions book. Chinese medicine's singular contribution was systematizing it early and prescribing regulation: using one emotion to master another (grief can master anger — compassion dissolves hot temper), shifting the mind (redirecting attention), quiet sitting and breath regulation. The summit of emotional cultivation is not having no emotions, but letting them flow without sediment.
Where the evil gathers, the qi must be weak. Wind, rain, cold and heat — meeting no deficiency, the evil cannot wound alone. — Where evils gather, qi must be weak; wind, rain, cold and heat, finding no weakness, cannot wound alone.— Huangdi Neijing · Lingshu · "Baibing Shisheng"
Causes are only the starting point; the doctrine of mechanisms studies the dynamics of imbalance: righteous and evil in contention — the tug-of-war between the body's defense-and-repair force and the pathogen decides the disease's advance or retreat ("where righteous qi abides within, evils cannot interfere" is Chinese medicine's immunology); yin-yang derangement — balance tipped any way becomes illness (yang excess = heat, yin excess = cold, yang deficiency = cold, yin deficiency = heat); and disorder of the qi-machinery — the rise-fall-exit-enter goes awry (stomach qi that should descend instead rises = vomiting; clear yang that should rise fails to = dizziness).
Chinese medicine pays special attention to the "direction" of disease: external illness enters from surface to interior (skin → flesh → organs), from light to grave; treatment's meaning lies in "intercepting and turning" — inviting the evil out at the shallow layer before it goes deep. That is why the sweat method is applied early in colds, and why "treating the not-yet-ill" is ranked the highest principle. Once you grasp mechanisms, you understand why Chinese doctors ask so many questions: cold or heat preference? Where do you sweat? Thirsty or not? Stool texture? — every question fixes a coordinate for locating the imbalance.