In Chinese philosophy, things in nature can be classified in 5 types: wood, fire, earth, metal, water. Most Westerners fail to understand the concept because they tend to take these names literally. These five elements are not just the matters that the names refer to, but rather metaphors for describing how things interact and relate to each others.
It is believed that wood produces fire; fire produces earth; earth produces metal; metal produces water; water produces wood. On the other hand, wood controls earth; earth controls water; water controls fire; fire controls metal; metal controls wood. These interactions and relationships forms a framework for different schools of philosophy, belief and discipline. The interaction of five elements becomes a tool that helps scholars sort out observations and empirical data. Based on observations of how things interact, things are classified into one of the five elements so that they fit into the observed pattern. Then one can draw high level conclusions or predictions based on the types.
In Chinese medicine, liver, tendon, eyes are wood type; heart, blood vessels, tongue are fire type, spleen, muscle, mouth are earth type; lungs, skin, hair, nose are metal type; kidney, bone, ears are water type etc. This classification helps in adjusting the balance in the body.
The study of herbal therapy classified the properties and effects of each herbs according to empirical observations on how the herbs affects the body. For example, if one herb causes dry mouth and chapped skin, it would be classified as "fire" type. The element type of the herbs can serve a useful purpose when designing a herbal cocktail remedy because the "fire" ingradient can be controlled by adding some "water" ingredients; or addition of "metal" ingredients can assist the "water" ingredients to do its job in controlling the "fire" etc. A "water" type herb or food is beleived to benefit a "wood" type organ etc. The principle of the five elements are used extensively in Chinese medicine.
The study of Feng Shui focuses on how the five elements type of people, objects, landscapes etc. affects the harmony of the environment. Even Chinese astrology are based on the five elements. The model is used in attempts to make sense out of the unknowns, such as fortune telling and many other things. Of course, if there is not a pattern in the data, the result is unreliable regardless how good the sorting tool is.
At first sight, the theory behind the five elements seems to be very unscientific. However, if one thinks of it as a model and scaffolding to build knowledge on, one may find the system intriguing. Even the western scientific model does not have to be valid to be useful. One example is the electric current, it was defined as positive charge flowing in a electric circuit. Even when we know now the electrons, not the positive charge flow in the circuit, the definition of electric current is not abandoned because the model still works despite the invalid underlying theory. By the same token, the five elements system is a useful model regardless how unscientific it seems to be.
Herbal therapy works wonder for certain type of diseases, and is widely used in China. Accupunture was proven effective in suppressing certain types of symptoms even though medical equipment fails to show how qi flows in the meridian of the body. If the model works, it does not have to map to the real physiology.
The principle of the five elements have a deep root in the Chinese culture.