I’m a bit confused by the following statement
To further categorize, the chest area is considered yang, while the abdomen is yin. The Heart and Lungs are therefore yang types of zang organs. The Heart is yang within yang, while the Lungs are yin within yang. Below the diaphragm in the abdomen we have the yin zang organs: Liver, Spleen and Kidneys. The Kidneys are yin within yin. The Liver is yang within yin, and the Spleen is utmost yin within yin. This classification helps one to understand the relative relationships between the organs and the body as a whole in terms of location, function, and nature of each organ. – THE TRUTH FROM THE GOLDEN CHAMBER – SuWen4
Why is the Spleen classified as ‘utmost yin within yin’?
In SuWen4 (as well as LingShu1) the Zang are categorised in relation to the seasonal evolution of Yin and Yang which mirrors their position in the body as in the figure below:
The young Yang (Yang within Yin) of the Spring/Liver comes into fruition with the full manifested Yang of the Summer/Heart (Yang within Yang) which declining into the Autumn/Lung (Yin within Yang) establishes the beginning of the Yin (the young Yin) which in turn evolves in the full developed Yin of the Winter/Kidney (Yin within Yin). In the centre the Spleen, associated with the dampness of the late-summer, is depicted as the utmost Yin (ZhiYin) which represents the ability of receiving and responding, and in so doing it gives form and make life appear through transformations. Therefore, the use of the character ZhiYin 至陰 confirms the adoption of the idea that the main feature of Yin is that of receiving, holding and transforming the Yang (role that is epitomised by the Earth and Post natal Qi). Yin represents, in this context, the Earth as matter able to give form to the 10000 creatures through the activation from Heaven/Yang/function– rather than cold which is basically absence of Yang. This logic is employed also when the 6 conformations are named: in fact, the use of the term TaiYin as greater/major-Yin may seem at first inadequate when considering that ShaoYin should be more Yin than TaiYin. However, TaiYin was imagined as representing the Earth as matter and essence of post-natal Qi (Lu/Sp).