四损由人事，四不足由天禀；四损在临时，四不足在平素。然四不足亦有由四损而来者，不可以四损之外，便无
不足。四不足者，气、血、阴、阳也。气不足者，少气不足以息，语言难出也，感邪虽重，反不成胀满，痞塞，凡
遇此证，纵宜宣伐，必以养气为主。血不足者，面色萎黄，唇口刮白也，感邪虽重，面目反无阳色，纵宜攻利，
必以养血为主。阳不足者，或四肢厥逆，或肌体恶寒，恒多泄泻，至夜益甚，或口、鼻冷气，受邪虽重，反无发
热、苔刺、燥渴，凡遇此等证，纵欲攻利清热，必先之以温补，待其虚回，实证全见，然后以治实之法治之。阴不
足者，自然五液枯干，肌肤甲错，感邪虽重，应汗无汗，应厥不厥，遇此等证，纵宜攻利，必先之以养阴，待其气
化津回，邪多不治自退。设有未退，酌用清利，攻利若
早，其病益甚。
以上四不足，合前条四损，总不可正治其邪。必以养正为要，先服养正药，待其实证悉见，方可攻邪。若服攻
邪药，虚证复见，仍当调补其虚，养正以和邪，去邪以安正，互相加减，迭为进退，直待邪尽去而正不伤，方为善治。
三复者，劳复、食复、自复也。劳复者，大病后因劳碌而复也。不必大费气力，即梳洗、沐浴亦能致复。复则
复热，诸证复起，惟脉不沉实为辨。轻者静养自愈；重者必大补，以调其营卫，和其脏腑，待其表里融和方愈。误
用攻下、清凉，必致不救，安神养血汤主之。若因饮食过多而复者，舌苔必复黄，轻者损谷自愈，重则消导始愈。
若无故自复者，乃伏邪未尽也，当问从前所见何证，服何药而解，今仍用前药，以涤其余邪则愈。时疫复证有复至
再三者，屡复之后，必兼四损、四不足证，宜参前条加减进退之法治之。
四损是由人为因素造成的，四不足则是先天禀赋所致；四损发生在疾病发作期间，四不足则体现在平素体质上。然而，四不足也有可能是由四损发展而来的，不能认为在四损之外就没有不足的情况。所谓四不足，指的是气、血、阴、阳的不足。
气不足的表现是：气息微弱，呼吸短促，说话无力，感邪即使严重，反而不会出现胀满、痞塞的症状。凡是遇到这种情况，纵然适宜使用宣散攻伐之法，也必须以补气为主。
血不足的表现是：面色萎黄，嘴唇和口唇苍白。感邪即使严重，面目反而没有阳热之色。纵然适宜使用攻下利导之法，也必须以养血为主。
阳不足的表现是：有的四肢厥冷逆乱，有的肌肤怕冷，常常伴有腹泻，到了夜间症状加重，或者口鼻呼出冷气。受邪即使严重，反而没有发热、舌苔生刺、燥渴等症状。凡是遇到这类证候，纵然想要使用攻下利导或清热之法，也必须先使用温补之法，待其虚象恢复，实证完全显现后，然后再用治疗实证的方法来处理。
阴不足的表现是：自然出现五液枯干，肌肤粗糙如鳞甲交错。感邪即使严重，应当出汗却无汗，应当出现厥逆却不出现厥逆。遇到这类证候，纵然适宜使用攻下利导之法，也必须先使用养阴之法，待其气化功能恢复、津液回流，病邪大多不药自退。假如病邪未退，酌情使用清利之法。如果过早使用攻下利导之法，病情反而会加重。
以上四种不足，结合前条所述的四种损伤，总的原则是不可直接正治其邪气（即不可单纯针对邪气进行常规治疗）。必须以扶正养正为关键，先服用扶正的药物，待其实证全部显现后，才可以攻逐邪气。如果服用攻邪药物，虚证再次出现，仍应当调补其虚损。通过扶正来调和邪气，祛邪来安护正气，两者互相加减，交替进退，直到邪气完全去除而正气不受损伤，这才是善于治疗。
所谓的“三复”，是指劳复、食复、自复。劳复，是指大病之后因劳累而复发。不必耗费很大的气力，即使是梳洗、沐浴也能导致复发。复发时会出现重新发热，各种症状再次兴起，只有脉象不沉实可以作为鉴别点。轻者静养即可自愈；重者必须大补，以调和营卫，和谐脏腑，等到表里融和才能痊愈。误用攻下、清凉之法，必然导致无法救治，主方为安神养血汤。如果是因饮食过多而复发的，舌苔必然会再次变黄，轻者减少饮食即可自愈，重者则需要消食导滞才能痊愈。如果是无故自行复发的，这是伏留的邪气未尽的缘故，应当询问从前所见的是什么证候，服用了什么药物而解除的，现在仍然使用之前的药物，以涤荡剩余的邪气就能痊愈。时疫的复证有复发两三次甚至更多的，多次复发之后，必然兼有四损、四不足的证候，应当参考前条所述加减进退的方法进行治疗。
The "Four Damages" (Si Sun) arise from human factors, whereas the "Four Deficiencies" (Si Bu Zu) stem from congenital endowment; the Four Damages occur during the acute phase of illness, while the Four Deficiencies are evident in one's baseline constitution. However, the Four Deficiencies may also originate from the Four Damages; one cannot assume that deficiencies do not exist outside the scope of damages. The Four Deficiencies refer to Qi, Blood, Yin, and Yang insufficiency.
Qi deficiency manifests as shortness of breath, difficulty in speech, and weak respiration. Even if the pathogenic factor is severe, distension or epigastric fullness (Pi Se) does not typically occur. In such cases, although dispersing and purging methods might seem indicated, the primary approach must be tonifying Qi.
Blood deficiency presents with sallow complexion and pale lips and mouth. Even with a severe pathogenic invasion, the face and eyes lack a yang-heat appearance. Although purgative and draining methods might appear suitable, the treatment must primarily focus on nourishing Blood.
Yang deficiency is characterized by cold extremities (Jue Ni), aversion to cold in the body, frequent diarrhea which worsens at night, or cold air exhaled from the mouth and nose. Despite severe pathogenic infection, there is typically no fever, prickly tongue coating, or dry thirst. For these syndromes, even if purgation, drainage, or clearing heat seems necessary, warming and tonification must be applied first. Once the deficiency recovers and the excess syndrome fully manifests, then treat the excess condition accordingly.
Yin deficiency naturally leads to the drying up of the five fluids and rough, scaly skin. Even with severe pathogenic infection, sweating may fail to occur when it should, or cold limbs may not appear when expected. In such cases, although purgation and drainage might be considered, nourishing Yin must be the initial step. Once Qi transformation recovers and body fluids return, the pathogen often retreats spontaneously. If the pathogen persists, clear and drain methods may be used judiciously. Premature use of purgatives and drainers will exacerbate the disease.
Regarding the above four deficiencies, combined with the previously mentioned four damages, the general principle is that one should not directly treat the pathogen using standard excess-clearing methods. The priority must be supporting the healthy Qi (Zheng Qi). Administer supportive medicines first; only when the excess symptoms fully manifest can one proceed to attack the pathogen. If attacking medicines cause the deficiency symptoms to reappear, one must revert to tonifying the deficiency. Support the healthy Qi to harmonize with the pathogen, and eliminate the pathogen to pacify the healthy Qi. These approaches should be adjusted and alternated until the pathogen is completely eliminated without damaging the healthy Qi; this constitutes skillful treatment.
The "Three Recurrences" (San Fu) refer to recurrence due to labor (Lao Fu), food (Shi Fu), and spontaneous recurrence (Zi Fu). Lao Fu occurs when a patient relapses after major illness due to overexertion. Even minor exertion, such as combing hair, washing, or bathing, can trigger recurrence. Upon recurrence, fever returns, and various symptoms re-emerge; the distinguishing feature is that the pulse is not deep and solid. Mild cases heal with rest; severe cases require strong tonification to regulate Ying and Wei (Defensive and Nutritive Qi) and harmonize the Zang-Fu organs. Recovery occurs only when interior and exterior are harmonized. Misuse of purgatives or cooling agents will lead to untreatable conditions; Anshen Yangxue Tang (Calming the Spirit and Nourishing Blood Decoction) is the principal formula.
If recurrence is caused by excessive food intake, the tongue coating will turn yellow again. Mild cases resolve by reducing food intake; severe cases require digestion-promoting and guiding-out methods. If recurrence happens spontaneously without obvious cause, it indicates that latent pathogens remain unresolved. One should inquire about the previous symptoms and the medications that resolved them, then reuse those same medications to cleanse the residual pathogens. Epidemic febrile diseases may recur two or three times. After multiple recurrences, symptoms of the Four Damages and Four Deficiencies often coexist. Treatment should follow the principles of adjustment and alternation described in the preceding section.