The theory of yin and yang emphasizes that everything in this universe has 2 sides. For example, day and night, hot and cold, male and female, winter and summer etc. Yin and yang are opposing but also one and the same. Yin transforms into yang and yang transforms into yin constantly. There is always yin within yang and yang within yin. The two can be divided into endless amounts but everything will always contain both yin and yang. 

In nature, yin and yang keeps everything in balance. These 2 forces are always changing and moving to keep everything in equilibrium. If these forces stagnate, then balance is thrown off and problems occur. 

Yin and Yang in Traditional Chinese Medicine

In Chinese medicine our bodies and minds are seen as a reflection of nature and what is going on in it. With that in mind, the balance of yin and yang within us and all our parts is crucial when it comes to our health.