The theory of Yin Yang is a generalization of the properties of the opposite sides of things or phenomena that are related to each other in nature. In Chinese, Yin literally is the same word with shadow, Yang is the same word as sunshine. This theory originated from the Tao philosophy from ancient Chinese times. As Lao Tsu said in his famous book Tao Te Ching: ‘The nameless (Tao) is the beginning of heaven and earth; The named is the mother of ten thousand things (all things exist)... These two spring from the same source but differ in name;...’ Huang Di Nei Jing, the bible of Chinese medicine states: "Yin and Yang are the way of heaven and earth, the discipline of all things, the parents of change, the beginning of life and death, and the house of gods".
Your personal philosophy, the way you make sense of the world around you, directly influences what you value in medicine and health. So the concept of health is quite subjective. Some people see that no need to go to hospital as healthy, while others think suboptimal conditions are unhealthy. According to the philosophy of Chinese medicine, health is the balance of Yin and Yang, the co-existence of the two opposite aspects in all things.
When the theory of Yin and Yang is used in medicine, not only the attributes of opposition and restriction apply, its dynamic nature of forever changing also needs to be considered. The opposition and restriction of Yin and Yang are observed in all things but they are not absolute but relative. Yin and Yang share the same root from the ‘nameless’ Tao, therefore the dynamic balance has to be maintained by both sides to achieve optimal health. If the overall attributes of things, the subjects or levels of comparison remain unchanged, their Yin and Yang attributes are fixed. But if the overall attributes of things, the subjects or comparison level change, their Yin and Yang also change accordingly. These changes are considered in all aspects in Chinese medicine, such as Yin and Yang can change into each other; or, Yin can get into Yang or vice versa; if either side is too strong, it could weaken the other side; if one side is full and controlled, nurture the other side could also benefit the whole, etc. It's a bit like the concept of homeostasis, but considered in a holistic way with whole and all things, from micro to macro scales. The status of Yin and Yang are not only inspected in the body of people, it is also inspected in the season, the weather, the geographic location, the climate and micro environment when diagnosing the conditions of a person. For example, In the process of disease development, the transformation of Yin and Yang often manifests as the tipped balance of cold and heat due to their transformation under certain conditions. From conventional medicine point of view, common flu is the virus infection of the body that causes temperate symptoms. Normally the doctor will not administer any medication, only recommend rest and increase fluid intake. From a Chinese medicine point of view, flu could be caused by Yin invading Yang, which is called cold flu, or Yang invading Yin, which is called hot flu. Different herbal teas will be recommended accordingly. Think about a scenario where on a hot summer day, you work outside and sweat a lot, all the pores on your body are open, then you walk into a heavily air conditioned room, you are prone to sneeze and catch cold. All the Yang/warmth are floating outwards through the pores on the skin, which allow the cold Yin air to get inside the body. It is like all the soldiers are rushing out of the city, the enemy takes the chance to occupy the empty city. It is very different with the situation where on a cold winter day, you have to stay outside too long, couldn’t warm yourself up and eventuated a flu.
Chinese medicine is more of an art based on solid philosophical thinking. It is all about living with wholeness, living in an optimal state, living with contentedness and living in harmony with your elements. There is a saying that conventional medicine makes you live longer with modern technology; Chinese medicine makes you live healthier and happier with the empirical art of healing.