Simple to understand, Yin represents cold and Yang represents heat. If you're a Yin deficiency, you'll be cold and weak, and your body will be cold, such as cold hands and feet, and a thin white tongue; if you're a Yang deficiency, you'll be hot and weak, and your body will be hot, and your hands and feet will be hot, and you'll have a dry mouth, and a thick and yellow tongue. As long as the kidney is deficient will have back pain, so you can't use back pain to distinguish whether it is kidney yin deficiency or yang deficiency. In general, there will not be the presence of both diseases.