Essence, spirit, blood and qi are the fundamental and indispensable elements comprising life in a human being. Their individual qualities and interactions with one another determine the pattern for all movements of life, reflecting those of the cosmos. This book explores these ideas, considers each of the substances and presents observations and insights which are rooted in scholarship and philosophical reflection. The book begins with a description of duality and oneness in the Chinese system, beginning with yin and yang, heaven and earth, and going on to essence and spirit, blood and qi. There follows a section which looks more closely at the jing (essences), and goes on to describe their relationship to the tastes (jingwei), with the spirit (jingshen) and the relationship between jing and shen. The second part of the book is a presentation of qi and blood and their inter-relationships looked at from the classical texts. Sprinkled with classical quotes, and with a clear index, this book serves as an essential reference book for these basic concepts of Chinese medicine.