二十难曰∶经言脉有伏匿，伏匿于何脏而言伏匿耶？
然∶谓阴阳更相乘，更相伏也。脉居阴部，
而反阳脉见者，为阳乘阴也；脉虽时沉涩而短，此谓阳中伏阴也；脉居阳部，而反阴脉见者，为阴乘
阳也，脉虽时浮滑而长，此谓阴中伏阳也。
此言阴阳相乘中又有伏匿之义也。经言无考。伏匿者，谓不见于本位，反藏于他部而见脉也。脉
之阴阳，非独言寸为阳，尺为阴也。若以前后言之，即寸为阳部，尺为阴部；若以上下言之，肌肉上
为阳部，肌肉下为阴部。阳乘阴者，尺中已浮滑而长，又时时沉涩而短，故曰阳中伏阴，言阳虽乘阴
而阴犹伏于阳内也。阴乘阳者，寸关已沉短而涩，又时时浮滑而长，故曰阴中伏阳，言阴虽乘阳而阳
犹伏于阴中也。
重阳者狂，重阴者癫。脱阳者见鬼，脱阴者目盲。
此又因阴阳之伏匿而极言之。重阳重阴，言不止伏匿，而阴皆变为阳，阳皆变为阴也。狂者阳
疾，癫者阴疾。重阳者狂，木火之阳旺也。重阴者癫，金水之阴旺也。心主喜，肝志怒，狂者木火
有余，故多喜怒。肾志恐，肺主悲，癫者金水有余，故多悲恐。脱阳者阴旺，鬼，阴类也，故见之。
脱阴者，肝窍于目，肝藏血，血舍魂，魂化神，魂神升发而生光明，上开双窍，则为两目，阴者阳
之宅也。阴脱宅倾，神魂散亡，是以目盲，名虽阴脱，而实脱阴中之阳气也。
第二难说：医经中提到脉象有“伏匿”的情况，那么这种“伏匿”是相对于哪一脏而言的呢？
回答：是指阴阳之气相互乘袭、相互潜伏的现象。如果脉象出现在阴部（尺部），却反而出现阳脉（如浮、滑、长等脉象），这叫做“阳乘阴”；此时脉象虽然有时表现为沉、涩、短，但这正是“阳中伏阴”的表现。如果脉象出现在阳部（寸部），却反而出现阴脉（如沉、涩、短等脉象），这叫做“阴乘阳”；此时脉象虽然有时表现为浮、滑、长，但这正是“阴中伏阳”的表现。
这里所说的阴阳相乘之中，又包含了“伏匿”的含义。医经原文对此没有详细考证。“伏匿”的意思是，脉象不显现于其本来的部位，反而隐藏在其他部位而表现出来。脉象的阴阳划分，不仅仅指寸为阳、尺为阴。如果以前后位置来说，寸部为阳部，尺部为阴部；如果以上下位置来说，肌肉之上为阳部，肌肉之下为阴部。所谓“阳乘阴”，是指尺部（阴部）已经出现浮、滑、长的阳脉，却又时时出现沉、涩、短的阴脉，所以称为“阳中伏阴”，意思是阳气虽然乘袭了阴位，但阴气仍然潜伏在阳气之中。所谓“阴乘阳”，是指寸、关部（阳部）已经出现沉、短、涩的阴脉，却又时时出现浮、滑、长的阳脉，所以称为“阴中伏阳”，意思是阴气虽然乘袭了阳位，但阳气仍然潜伏在阴气之中。
阳气重叠亢盛则发为狂证，阴气重叠亢盛则发为癫证。阳气脱失者会看见鬼怪，阴气脱失者会双目失明。
这是进一步因阴阳之气的伏匿现象而进行的极端推论。“重阳”和“重阴”，指的是不仅限于伏匿，而是阴完全转化为阳，阳完全转化为阴。狂证属于阳疾，癫证属于阴疾。“重阳者狂”，是因为木火之阳气旺盛。“重阴者癫”，是因为金水之阴气旺盛。心主喜，肝志怒，狂证患者木火有余，所以容易喜怒无常。肾志恐，肺主悲，癫证患者金水有余，所以容易悲伤恐惧。阳气脱失时阴气偏旺，鬼怪属于阴类，所以患者会看见鬼怪。阴气脱失时，肝开窍于目，肝藏血，血舍魂，魂化生神，神魂升发而产生光明，向上开启双眼，即为两目。阴是阳的宅舍，阴气脱失则宅舍倾覆，神魂散亡，因此导致双目失明。名义上是阴气脱失，实际上是脱失了阴中之阳气。
Difficulty 20 states: The classics mention that pulses can be "hidden" (Fu Ni). Regarding which Zang organ is this "hiding" discussed?
The answer is: It refers to the mutual overacting and mutual hiding of Yin and Yang. When a pulse appears in the Yin position (Chi position) but unexpectedly shows Yang pulse characteristics (such as floating, slippery, or long), it is called "Yang overacting on Yin"; although the pulse may occasionally appear deep, choppy, and short, this is referred to as "Yin hidden within Yang". When a pulse appears in the Yang position (Cun position) but unexpectedly shows Yin pulse characteristics (such as deep, choppy, or short), it is called "Yin overacting on Yang"; although the pulse may occasionally appear floating, slippery, and long, this is referred to as "Yang hidden within Yin".
This statement implies that within the mutual overaction of Yin and Yang, there is also the meaning of "hiding". The classics do not provide further verification for this. "Hidden" means that the pulse does not manifest in its original position but instead hides in another position and manifests there. The Yin-Yang classification of pulses is not solely defined by Cun being Yang and Chi being Yin. If described by anterior-posterior position, Cun is the Yang position and Chi is the Yin position; if described by superior-inferior position, the area above the muscle is the Yang position, and the area below the muscle is the Yin position. "Yang overacting on Yin" means that at the Chi position (Yin position), Yang pulses such as floating, slippery, and long are already present, yet they occasionally show Yin pulses such as deep, choppy, and short. Thus, it is called "Yin hidden within Yang," indicating that although Yang overacts on Yin, Yin still hides within Yang. "Yin overacting on Yang" means that at the Cun and Guan positions (Yang positions), Yin pulses such as deep, short, and choppy are already present, yet they occasionally show Yang pulses such as floating, slippery, and long. Thus, it is called "Yang hidden within Yin," indicating that although Yin overacts on Yang, Yang still hides within Yin.
Excessive Yang leads to mania (Kuang); excessive Yin leads to melancholic psychosis (Dian). Loss of Yang results in seeing ghosts; loss of Yin results in blindness.
This further elaborates on the extreme implications of the hiding of Yin and Yang. "Excessive Yang" and "Excessive Yin" refer not only to hiding but to the complete transformation of Yin into Yang and Yang into Yin. Mania is a Yang disease, while Dian is a Yin disease. "Excessive Yang leads to mania" indicates the prosperity of Wood-Fire Yang. "Excessive Yin leads to Dian" indicates the prosperity of Metal-Water Yin. The Heart governs joy, and the Liver governs anger; patients with mania have excess Wood-Fire, hence they exhibit frequent mood swings between joy and anger. The Kidney governs fear, and the Lung governs sorrow; patients with Dian have excess Metal-Water, hence they exhibit frequent sorrow and fear. When Yang is lost, Yin becomes prosperous; since ghosts belong to the Yin category, the patient sees them. When Yin is lost, considering that the Liver opens into the eyes, the Liver stores Blood, Blood houses the Hun (Ethereal Soul), and Hun transforms into Shen (Spirit), whose ascending and generating power creates brightness, opening upwards into two apertures to form the two eyes. Yin is the residence of Yang. When Yin is lost, the residence collapses, causing the Hun and Shen to scatter and perish, resulting in blindness. Although named as loss of Yin, it is actually the loss of Yang Qi within Yin.