It is written that the ancient Chinese developed this theory based on thousands of years of observation and experience. The intricacies of nature and the physical world in which 'man' is an integral part and has struggled with since antiquity.The microcosm within the macrocosm. By definition Yin & Yang theory is the mutual interdependence of two opposites which only exist as a whole. This constant flux is readily measurable observable at microscopic and universal scales.(What example/s come to your mind ?) In Chinese medicine and its related natural sciences this is the 'ultimate theory ' that has not altered in over 2000 years.(leading to eventual stagnation of new ideas...) Maintaining this dynamic equilibrium between Yin and Yang is said to prolong health and vitality and can probably be applied to all other physical phenomena that is observable or measurable in modern science as we search deeper into ourselves our world and the universe. However does this ancient theory still serve a purpose and is it still practical and relevant to our lives in the 21st Century?