In the context of Cold Damage (Shanghan) disease, when there is fullness in the lower abdomen, how should this be elucidated? The term "fullness in the lower abdomen" refers to a sensation of distension below the umbilicus. The lower abdomen is governed by the Lower Jiao (Lower Burner). The *Nan Jing* (Classic of Difficult Issues) states: The Lower Jiao is located at the opening of the bladder and is primarily responsible for separating the clear from the turbid; its therapeutic domain is below the umbilicus. When pathogenic factors descend from above to the Lower Jiao, they condense and obstruct, failing to flow freely, thus causing fullness in the lower abdomen. Fullness in the chest or below the heart is caused solely by Qi stagnation, meaning there are no tangible accumulations. In contrast, abdominal fullness may be caused by dry feces. However, fullness in the lower abdomen is not merely due to Qi; it necessarily involves the accumulation of tangible matter. This is because the upper half of the body corresponds to the Yang of Heaven, where Clear Yang ascends; the lower half corresponds to the Yin of Earth, where Turbid Yin descends. Clear substances reside above, while turbid substances reside below. As stated in the *Nei Jing* (Inner Canon): Clear Yang exits through the upper orifices, and Turbid Yin exits through the lower orifices. When the turbid Yin that should be expelled fails to exit, it accumulates and causes fullness. Therefore, fullness in the upper part is caused by Qi, while fullness in the lower part is caused by tangible matter. The so-called "matter" refers to urine and blood. When pathogenic factors gather in the Lower Jiao, body fluids cannot circulate freely, and Qi and Blood cannot move smoothly. Consequently, either urine or blood remains stagnant in the lower region, generating distension, hardness, and pain. If the entire area from below the heart to the lower abdomen is hard, full, and painful, this indicates an excess of pathogenic factors, requiring the use of Da Xian Xiong Tang (Major Intractable Chest Decoction) to purge it. If only the lower abdomen is hard, full, and painful, with free urination, this is a syndrome of Blood Stasis (Xuexue); if urination is difficult, it is a syndrome of Urinary Retention (Niaose). The classics state: "When the lower abdomen is full, urination should be difficult; if it is now free, it indicates the presence of blood." It also states: "If the lower abdomen is hard and urination is difficult, it indicates the absence of blood." If urination is free and the patient exhibits mental agitation resembling madness, this is a definitive sign of a Blood Syndrome. Cases where urination is free but the lower abdomen is full indicate that the Taiyang (Greater Yang) pathogen has followed the meridians into the interior, resulting in blood stasis within the interior; this represents the Taiyang channel transmitting directly into the Bladder Fu organ. The classics state: "If Taiyang disease is not resolved, heat condenses in the Bladder, and the person becomes mentally agitated like a madman. If blood passes spontaneously, the condition resolves. If the exterior symptoms have not yet resolved, attack is not yet appropriate; one must first resolve the exterior. Once the exterior is resolved, if there is only urgency and binding in the lower abdomen, then attack is permissible, using Tao Ren Cheng Qi Tang (Peach Kernel Decoction to Direct the Qi)." Thus, it can be known that hardness and fullness in the lower abdomen result from the accumulation of tangible matter in the lower region. Employing methods of leaching or promoting diuresis, and adjusting according to circumstances, one can become a superior physician.