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In Chinese Medicine, we talk all the time about yin and yang, and try to categorize an illness into either one. 《素问·阴阳应象大论》 says “善诊者，察色按脉，先别阴阳,” telling us that the first thing you do for diagnosis if to decide on either yin or yang. 《类经·阴阳类》 says “认知疾病，… 必有所本，或本于阴，或本于阳，病变虽多，其本则一，” emphasizing the importance of locating the root as either yin or yang.

The thing with such a method is that you’re using a broad adjective to describe the situation. Coldness, the interior and deficiency are all yin-concepts, but speak of rather different situations. Coldness is about temperature; the interior refers to relative depth; while deficiency describes where you are on the power scale. If you’re cold inside and feel deficient, then it’s pretty straightforward, but then again we see people who feel cold on the outside, and that doesn’t fit. Also the case of the guy who feels weak but is feeling really hot and flustered. How do we classify that?

You then talk about what it is that is yin or yang, i.e. in terms of feeling cold or hot, she’s yin, but when it comes to the place where she’s feeling it, it’s on the skin, and so yang.