Diseases that present with fever and aversion to cold originate in the Yang aspect. Those without fever but with aversion to cold originate in the Yin aspect. Diseases originating in the Yang aspect typically resolve after seven days, while those originating in the Yin aspect typically resolve after six days. This is because seven is the number associated with Yang completion, and six is the number associated with Yin completion.

This section serves as a general outline summarizing Yin and Yang, and should ideally be placed at the beginning of the discussion on the Six Channels (Liu Jing). Since Wang Shuhe and subsequent scholars erroneously misplaced this text after the pulse and symptom patterns of the Taiyang channel, Yu Jia Yan interpreted "no fever" as "not yet having fever," which contradicts Zhang Zhongjing's original intent. Generally, when external pathogenic factors invade the body, the initial state of invasion is difficult to discern, but once symptoms manifest, they can be identified. By observing the manifestation, one can infer the nature of the invasion. Due to differences between Yin and Yang channels, manifestations differ in the presence or absence of fever, thereby determining the healing periods based on odd numbers for Yang and even numbers for Yin. "Originating in Yang" refers to pathogens entering Yang channels; "originating in Yin" refers to pathogens entering Yin channels. This aligns with the *Huangdi Neijing* (Yellow Emperor's Inner Canon) stating, "Yang excess leads to body heat, Yin excess leads to body cold," representing variations in Yin-Yang imbalance.

Fever accompanied by aversion to cold occurs in Taiyang disease, which resides on the exterior of the body where Ying (Nutrient) Qi and Wei (Defensive) Qi circulate. Ying and Wei Qi are transformed from the essence of food and drink. Wei Qi moves outside the vessels, originating from the True Yang of the Mingmen (Gate of Life) in the Lower Jiao. It steams the grain qi and distributes it to the skin, controlling the opening and closing of pores to secure the exterior. Ying Qi moves inside the vessels, originating from the True Qi of the Taiyin in the Upper Jiao. It descends the refined essence of middle Jiao grain qi to nourish, irrigate, and fill the entire body. If wind pathogen injures Wei Qi, then Wei Yang is affected, resulting in stiffness and pain in the head and neck, fever, aversion to wind, and sweating. If cold pathogen injures Ying Qi, then Ying Yin is affected, resulting in stiffness and pain in the head and neck, fever, aversion to cold, absence of sweat, body ache, vomiting, and regurgitation. Although Ying and Wei have depths of penetration, their manifestation in the Taiyang channel is consistent. Regarding the Yangming channel, it lies within the Ying and Wei, in the muscles and interstices. Although not directly concerning the superficial Ying and Wei, pathogens must still enter through them. Thus, some may experience no fever but only aversion to cold on the first day. Once the pathogen enters Yangming, aversion to cold ceases, body heat appears, spontaneous sweating occurs, and there is aversion to heat rather than cold. However, if there is profuse sweating yet slight fever and aversion to cold persist, the exterior syndrome has not yet resolved. If the pathogen enters the interior, tidal fever will develop. Therefore, if there is no tidal fever, Chengqi Tang (Cathartic Decoction) should not yet be used. Shaoyang (Lesser Yang) is the inner layer of the shell. From here outward, reaching Taiyang via Yangming, it belongs to the Yang aspect and is considered Exterior. Shaoyang is still the inner surface of the Three Yang channels, hence called Half-Exterior. From here inward lie the intestines, stomach, and zang-fu organs, belonging to the Yin aspect and considered Interior. Shaoyang acts as the framework surrounding the gastrointestinal organs, hence called Half-Interior. Its system attaches to the Exterior, while its location connects to the Interior. Pathogens attacking Shaoyang do not reside as superficially as in Taiyang. In Taiyang, aversion to cold can lead to fever, and fever can lead to aversion to cold; cold and heat often occur together. Only in Shaoyang does the pathogen penetrate deeply, making it difficult to exit to the skin and Ying/Wei. Thus, its manifestation resembles malaria, with alternating chills and fever. While the three Yang channels all feature fever, their presentations vary, yet fever is common to all. In contrast, the Three Yin channels generally do not present with fever; cold limbs and aversion to cold are the norm. Occasionally, there are atypical cases of fever, lack of aversion to cold, and warm hands and feet, which represent variations. For example, in Taiyang exterior syndromes, there is no fever. One text states that in Taiyin wind-stroke, with vexing pain in the four limbs, if the pulse is faint at the Cun position, choppy at the Chi position, but long, recovery is imminent. Another states that in Taiyin disease, if the pulse is floating, sweating methods may be used, preferably Guizhi Tang (Cinnamon Twig Decoction). Another states that in cold damage, if the pulse is floating and slow with spontaneously warm hands and feet, the disease relates to Taiyin. These are all Taiyin exterior syndromes without heat. Regarding Shaoyin disease, it is stated that initially there is atypical fever. The term "atypical" (fan) is used because fever appears when none is expected. This is an exterior syndrome of Shaoyin deficiency-cold, treated with Mahuang Fuzi Xixin Tang (Ephedra, Aconite, and Asarum Decoction). In Shaoyin syndrome, two or three days after onset, if the mouth feels normal and there is aversion to cold on the back, moxibustion should be applied, and Fuzi Tang (Aconite Decoction) administered. Similarly, in Shaoyin disease, with aversion to cold, curled-up posture, and diarrhea, or cold limbs; or in Shaoyin disease with cold limbs, aversion to cold, curled-up posture, absent pulse, restlessness without irritability—these are all aversion to cold without fever. Also, in Shaoyin disease with clear undigested stool in diarrhea, internal cold with external heat, cold limbs, and faint-to-absent pulse, if the body is atypically free of aversion to cold, this "lack of aversion" is atypical. The external heat and flushed face are not true fever but result from Yin excess repelling Yang to the exterior. In Shaoyin disease with diarrhea, if the diarrhea stops spontaneously, and although there is aversion to cold and a curled-up posture, the hands and feet are warm, treatment is possible. In Shaoyin disease, if the pulse is tight, and after seven or eight days spontaneous diarrhea occurs with the pulse suddenly becoming faint, hands and feet become warm, and the tightness disappears, resolution is near. In Shaoyin disease with vomiting and diarrhea, if hands and feet are not cold and there is atypical fever, death is not inevitable. Here, warm hands/feet and fever are termed "atypical." All these describe diseases originating in Yin without true heat. They are called non-fatal because Yin pathogens are receding and Yang Qi is returning, allowing for warming meridians and restoring Yang therapies. In Jueyin disease, fever is very common, while aversion to cold is rare, reflecting a balance of Yin and Yang, though Yang is prioritized. Thus, when there is heat, there is always cold limbs (Jue), and when there are cold limbs, there is heat. Fewer cold limbs and more heat indicate disease regression; more cold limbs and less heat indicate disease progression. However, fever with diarrhea, cold limbs, and restlessness is not true fever but indicates Yin excess and Yang exhaustion, separation of Yin and Yang, and is a fatal sign.

Since diseases originating in Yin inherently lack heat, any presence of heat is described as "atypical fever" or "atypical lack of aversion to cold." "Atypical" means something occurring that shouldn't, or failing to occur that should. All such true-cold/false-heat syndromes are listed among the Three Yin manifestations, underscoring that lack of heat is the basis of Three Yin diseases. When referring to "originating in Yin," what else could it refer to but the Three Yin channels? However, Cheng Wuji's old commentary claimed Yang equals heat and Yin equals cold, interpreting fever with aversion to cold as cold injuring Yang, and no fever with aversion to cold as cold injuring Yin. This fails to grasp Zhang Zhongjing's principles. Yu Jiayan followed Fang Youzhi, arguing wind is Yang, Wei is Yang, so disease starts in Yang; cold is Yin, Ying is Yin, so disease starts in Yin. This argument holds only if discussing pathogens confined to the Taiyang channel. Applying it to all Six Channels is a major error. Furthermore, interpreting "no fever with aversion to cold" as the initial stage before cold transforms into heat due to stagnation in Ying is flawed. If the brief period before transformation is called "no fever," Zhang Zhongjing's wording would be too ambiguous. If wind injuring Wei is "originating in Yang" and cold injuring Ying is "originating in Yin," then all "cold damage" in the Three Yang channels could be called "originating in Yin." Yet, some Three Yin sections begin with wind-stroke, which would imply "originating in Yang." Logically, this is inconsistent. The statement "Yang heals in seven days, Yin in six" describes a general principle, not an absolute rule. Seven is the number of Yang resurgence and Shaoyang generation; six is the number of Yin extremity and Old Yin completion. Yang numbers start at one and end at nine; Yin numbers start at two and peak at six, representing the ultimate numbers of Heaven and Earth. Within these extremes, Yin and Yang divide into Two Modes, which further divide into Four Images, meaning both Yin and Yang have "Less" and "More" aspects. Why choose Shaoyang (Less Yang) for Yang and Old Yin for Yin? Because Yang functions through "Less," while Yin reaches its extreme in "Old." Yang in its "Less" state generates life; Yin at its extreme gives birth to Yang. Thus, both serve as healing periods, reflecting the natural waxing and waning of Yin and Yang. Yang heals in seven days because Yang is active and changing, circulating on the exterior of meridians. After seven days, meridian qi exhausts the pathogen, as stated in *Suwen: Re Lun*: "For those not suffering from dual-channel cold attack, on the seventh day, Juyang (Taiyang) pathology declines, and headache improves." Yin heals in six days because Yin is static and stationary, unlike Yang channels that circulate fully. On the sixth day, Yin extremity allows Yang to return, as seen in Three Yin syndromes where warm hands/feet and heat overcoming cold lead to self-healing. Cases not healing on schedule require investigation into mismanagement, excessive pathogen, deficient healthy Qi, interior entry, organ involvement, or passing through channels, which cannot be bound by standard rules. Fever versus no fever is the source of differentiation; originating in Yang versus Yin is the foundation of treatment; odd/even numbers are the metrics of efficacy. Do these not encompass the beginning and end, summarize the Six Channels, and mark the core tenets of the entire treatise?

Reading Zhang Zhongjing's work reveals its correspondence with the *I Ching* (Book of Changes). The entire *I Ching* consists of odd and even lines; the entire *Shanghan Lun* consists of Yin and Yang. The six trigrams born from Qian and Kun correspond to the Six Channels derived from Yin and Yang. The depth, correctness, and variations of the Six Channels, appearing in complex layers and myriad forms, mirror the dynamic changes and numerical complexities of hexagrams and lines. The Sages, knowing words cannot fully express meaning, depicted principles through images, enabling people to explore principles via images. Zhang Zhongjing, knowing methods cannot cover all syndromes, divided Yin and Yang using cold and heat, enabling people to exhaust methods through differentiation. Thus, he first highlighted easily known cold/heat and easily distinguishable Yin/Yang, then elaborated on syndrome variations and methods, allowing for proper treatment. Syndromes are images; methods are principles. The 397 methods fall within the Six Channels; the Six Channels fall outside Yin and Yang. Only those deeply versed in the *I Ching* can truly understand Zhang Zhongjing's writings.