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Those with abundant Qi do not feel hot in the intense heat of summer, nor cold in the severe cold of winter; this is because their interior is solid, preventing the invasion of cold and heat pathogens. Those with weak constitutive Qi feel cold before it becomes cold, and feel hot before it becomes hot. Thus, if the weather remains cold and cloudy for ten days, they may contract cold-dampness leading to loose stools (Ru Xie); if the weather remains warm and sunny for ten days, latent heat may cause internal disorder. This is the normal state of those with weak constitutions. Generally, those who understand solar terms can distinguish the flourishing and decline, entry and exit of Yin, Yang, cold, and heat. The *Su Wen* states: "Those with Yang excess can endure winter but not summer; those with Yin excess can endure summer but not winter." This illustrates how human deficiency or excess interacts with cold and heat climates to produce favorable or unfavorable outcomes. At the beginning of Summer (Li Xia), the climate shifts to pure Yang, and all things flourish. When prescribing, one should "avoid heat when using hot remedies" (Yong Re Yuan Re). For instance, Guizhi Tang (Cinnamon Twig Decoction), Mahuang Tang (Ephedra Decoction), Da Qing Long Tang (Major Green Dragon Decoction), and other hot medicines must be combined with Zhi Mu (Anemarrhenae Rhizoma), Sheng Ma (Cimicifugae Rhizoma), and Shi Gao (Gypsum Fibrosum). In doing so, they are no longer purely hot medicines. At the beginning of Winter (Li Dong), both Yin energies are exhausted, and the qi combines into pure Yin. When prescribing, one should "avoid cold when using cold remedies" (Yong Han Yuan Han). For example, when taking various cooling agents, if diarrhea occurs excessively after taking the decoction, subsequent doses should be stopped without completing the full prescription. This means not fully utilizing cold medicines. Therefore, in cold months, one does not fully use cold medicines; if one did, the exterior and interior would both become cold, which the body cannot withstand. Treating variola (Chuang Zhen) should also follow this principle, adjusting medications according to deficiency/excess, cold/warmth, and the entry/exit of Yin/Yang. How can one rigidly adhere to a single syndrome and a single herb as the ultimate treatment? Patients with chronic excess or chronic deficiency should simply use ancient formulas without modification. At the Spring Equinox, Yang gradually moves outward while Yin gradually moves inward; at the Autumn Equinox, Yang gradually moves inward while Yin gradually moves outward. How can medication be applied uniformly? This involves aligning with the four seasons' Yin and Yang, accommodating the interior/exterior and deficiency/excess of the Zang-Fu organs. Thus, when Yang exits and Yin enters, excess syndromes resolve; when Yin exits and Yang enters, deficiency syndromes resolve. Therefore, "Yang exiting and Yin entering" characterizes deficiency diseases, while "Yin exiting and Yang entering" characterizes excess diseases; this is the unchanging constant principle. For example, in a warm winter, deficiency patients are comfortable while excess patients fall ill; in a cold summer, excess patients are comfortable while deficiency patients fall ill. These are all caused by the changes in solar terms, Yin/Yang, and deficiency/excess. If winter is abnormally warm or summer abnormally cold—conditions not dictated by normal solar terms—one can avoid illness through proper self-care. Therefore, when Qian Yi treated variola, stating that spring and summer follow the norm while autumn and winter reverse it, he was discussing the meaning of Yin/Yang entry/exit and its accommodation of the exterior and interior.