经云。亢则害。承乃制。此言太过之害也。亢者过于上。而不能下。承者。受也。亢极则反受制也。如火本克金。
克之太过。则为亢。金之子为水。可以制火。乘火之虚。来复母仇。而火反受其制矣。在脉有之。阳实者脉必洪大。至
其极也。脉反匿伏。阳极似阴也。阴虚者。脉必细微。至其极也。脉反躁疾。阴极似阳也。凡过极者反兼胜己之化。是
皆阴阳亢制之理。惟明者知之。
伤寒中风。虚损疟痢等病。人不常有。其朝夕失调。动辄得之。无过风食气三者。最宜辨晰。男女左关洪为感风。
肝脉沉伏亦感风。以风邪外束。故沉伏也。亦有中气不清。肝脉如不动者。血少也。男女右关短。为伤食。脾脉沉
伏。亦伤食。以脾虚食压。不能动也。亦有脾家湿而脉伏者。又脉沉食轻。脉短食重。此风与食之辨也。如肝脉浮洪甚。
脾脉略浮短。谓肝脉盛于脾。先得风而后得食也。脾脉洪短甚。肝脉略浮洪。谓脾脉盛于肝。先得食而后得风也。若肝
脉如不动。脾脉或短或洪盛。或二关俱洪。皆风食。惟肝脉伏。脾脉亦伏。余脉又无力。是为中气不清。胸中如云雾。
然中气不清。由脾胃不好致之须清中气。兼补脾。或余脉有力。切牙作难过之声。又是风食症。如肺脉洪。脾脉伏与短。
必气食相感。主大便不通。肺伏亦然。肝脉洪亦有感气者。然肺感轻。肝感重。凡病肝脉洪者。多感风也。惟肝脉细而
不续。胸中有时迷闷。有时清爽。此气郁也。
《内经》中说：“亢则害，承乃制。”这是说过度（亢盛）带来的危害。所谓“亢”，是指气机过于上行，而不能下行；所谓“承”，是承受、制约的意思。当某一行亢盛到极点时，反而会受到其所不胜之行的制约。例如，火本应克制金，但如果克制得太过，就称为“亢”。金的子行是水，水可以制约火，趁火之气虚弱之时，来报复母辈被克的仇怨，于是火反而受到水的制约了。在脉象上也有这种表现：阳气实盛的，脉象必然洪大，到了极点时，脉象反而隐匿伏匿，这是阳极似阴的表现。阴气虚衰的，脉象必然细微，到了极点时，脉象反而躁动急疾，这是阴极似阳的表现。凡是事物发展到极端的，往往会反过来呈现出克制它的那一行的特征，这都是阴阳亢盛与制约的道理，只有明理的人才能知晓。
伤寒、中风、虚损、疟疾、痢疾等疾病，人并不常患。日常生活中因起居饮食失调而导致的疾病，大多不过是风邪、食积、气滞这三者，最应当仔细辨别。男女患者左关脉洪大的，是感受风邪；肝脉沉伏的，也是感受风邪，因为风邪外束肌表，导致脉气沉伏。也有中焦清气不升，肝脉表现为不动（微弱或难以触及）的，这是血少的缘故。男女患者右关脉短的，是伤食；脾脉沉伏的，也是伤食，因为脾气虚弱，食物停滞压迫，导致脉气不能鼓动。也有脾脏湿盛而导致脉伏的情况。此外，脉沉说明食积轻，脉短说明食积重，这是风邪与食积的鉴别要点。如果肝脉浮洪非常显著，而脾脉略微浮短，这说明肝脉盛于脾脉，是先感受风邪而后又伤食。如果脾脉洪短非常显著，而肝脉略微浮洪，这说明脾脉盛于肝脉，是先伤食而后又感受风邪。如果肝脉表现为不动，脾脉或者短或者洪盛，或者左右两关脉都洪大，这都属于风邪和食积。只有当肝脉伏匿，脾脉也伏匿，其余脉象又无力时，这才是中气不清，患者感觉胸中如云雾般迷蒙。然而，中气不清是由脾胃功能不好引起的，需要清利中气，兼补脾气。如果其余脉象有力，且患者切牙发出难受的声音，这又是风食症。如果肺脉洪大，脾脉伏匿或短，必定是气与食相互感应，主要症状是大便不通；肺脉伏匿也是如此。肝脉洪大也有感受气滞的情况，但肺受气滞影响较轻，肝受气滞影响较重。凡是病见肝脉洪大的，多是由于感受风邪所致。只有当肝脉细弱且不连续，胸中有时迷闷、有时清爽时，这才是气郁证。
The *Classic of Internal Medicine* states: "Excess leads to harm, and control arises from succession." This refers to the harm caused by excessiveness. "Excess" (Kang) means qi moves excessively upward and fails to descend; "Succession/Control" (Cheng) means to bear or restrain. When a phase becomes excessive to the extreme, it is counteracted by the phase that overcomes it. For instance, Fire normally controls Metal. If this control is excessive, it is termed "Excess." The son of Metal is Water, and Water can control Fire. Taking advantage of the deficiency of Fire, Water avenges the injury done to its mother (Metal), causing Fire to be controlled by Water. This principle is also reflected in pulse diagnosis. When Yang is full and solid, the pulse must be surging and large; when it reaches the extreme, the pulse may become hidden and deep, which is known as "Extreme Yang resembling Yin." When Yin is deficient, the pulse must be fine and minute; when it reaches the extreme, the pulse may become restless and rapid, which is known as "Extreme Yin resembling Yang." Generally, when things reach an extreme, they tend to manifest characteristics of the phase that overcomes them. These are all principles of Yin-Yang excess and mutual control, understood only by the enlightened.
Diseases such as Cold Damage (Shanghan), Wind Strike (Zhongfeng), deficiency consumptions, malaria, and dysentery are not commonly contracted by people daily. However, due to irregular daily routines and diet, one often suffers from disorders involving wind, food stagnation, or qi stagnation. These three factors require careful differentiation. In both men and women, a surging pulse at the left Guan position indicates wind invasion. A deep and hidden Liver pulse also indicates wind invasion, as external wind pathogens constrain the exterior, causing the pulse to sink and hide. There are cases where the clear qi of the middle jiao is turbid, and the Liver pulse appears motionless (weak or imperceptible); this is due to blood deficiency. In both men and women, a short pulse at the right Guan position indicates food injury. A deep and hidden Spleen pulse also indicates food injury, because Spleen Qi is deficient and food stagnation suppresses it, preventing the pulse from moving vigorously. There are also cases where dampness in the Spleen causes a hidden pulse. Furthermore, a deep pulse suggests mild food stagnation, while a short pulse suggests severe food stagnation; this distinguishes wind from food. If the Liver pulse is floating and surging significantly, while the Spleen pulse is slightly floating and short, it means the Liver pulse dominates the Spleen pulse, indicating wind was contracted first, followed by food injury. If the Spleen pulse is surging and short significantly, while the Liver pulse is slightly floating and surging, it means the Spleen pulse dominates the Liver pulse, indicating food injury occurred first, followed by wind contraction. If the Liver pulse appears motionless, and the Spleen pulse is either short or surging and full, or if both Guan positions are surging, these are cases of wind and food. Only when the Liver pulse is hidden, the Spleen pulse is also hidden, and other pulses are forceless, does this indicate turbidity of Middle Qi, with the patient feeling chest congestion like clouds and mist. However, turbidity of Middle Qi results from poor Spleen and Stomach function; treatment requires clearing Middle Qi and supplementing the Spleen. If other pulses are forceful, and the patient grinds their teeth making uncomfortable sounds, this is again a wind-food syndrome. If the Lung pulse is surging, and the Spleen pulse is hidden or short, there is likely mutual interaction between qi and food, primarily manifesting as constipation; a hidden Lung pulse presents similarly. A surging Liver pulse can also indicate qi stagnation, but lung qi stagnation is mild, whereas liver qi stagnation is severe. Generally, diseases presenting with a surging Liver pulse are mostly due to wind invasion. Only when the Liver pulse is fine and discontinuous, and the chest feels intermittently congested and intermittently clear, is this diagnosed as Qi Depression (Qi Yu).