The Five Elements or Forces (wu hsing)—which, according to the Chinese, are metal, air, fire, water, and wood—are first mentioned in Chinese literature in a chapter of the classic Book of History.6 They play a very important part in Chinese thought: ‘elements’ meaning generally not so much the actual substances as the forces essential to human, life. They have to be noticed in passing, because they were involved in the development of the cosmogonical ideas which took place in the eleventh and twelfth centuries A.D. Page 85