It is stated in The Medical Classic of the Yellow Emperor: "Human life originates in the qi of Heaven and Earth, and develops according to the normal order of the four seasons." That is to say, human life is also endowed with the qi of nature. Qi is the material basis on which life activities are maintained. Various actions and changes of qi accompanied by transformation of energy are the fundamental characteristics of life activities. Without the actions and changes of qi there would be no life activities.
Originally, qi was not a philosophical concept. It meant air, as it is commonly used in the everyday Chinese language. Its meaning was extended to the philosophical field, serving as a symbol for the primordial substance that was thought to constitute the universe. It was believed to be invisible, taking the form of particles, constantly moving and changing, and giving rise to energy and activities. According to ancient Chinese philosophy, the universe originates from Taiji, a synonym for original or primitive qi. Its essential part, or essential qi, was thought to be the basic element out of which the universe is composed, and everything in the world is produced through the movement and changes of qi.
Since qi is invisible and acts as the motive force for all kinds of life activities, the word qi in Chinese medicine is often translated into English as "vital energy". This, however, only refers to qi in its narrow (physiological) sense, and the world "energy" is comprehended from the viewpoint of modern physics, i.e., matter and energy are regarded as equivalents, mutually convertible.
It is important to note that, in Chinese medicine, all life activities, including mental activities such as thought, will, and emotions, are based on the actions and changes of qi. Since qi is material, the theory of qi is a kind of materialism which explains mental activities in terms of objective matter. In addition, human beings are derived from the qi of nature, so the material world is primary, existing independent of the mind, while human beings are merely a part of nature. Based on this view, Chinese medicine emphasizes that all physiological activities are in conformity with the changes of the natural environment, and so, to preserve health one must keep in harmony with the law of nature.
Actually, qi in Chinese medicine can be classified into two main categories: the genuine or normal qi and the evil or pathogenic qi. The genuine qi of human beings comes from three sources: primordial qi, qi of food essence and air. Primordial qi is inborn, inherited from parents, while that qi of food essence and air are acquired. Genuine qi includes all the materials and energy essential for life activities as well as the body's resistance against disease.
Evil qi is harmful to health. It includes various pathogenic agents or factors, for example, abnormal atmospheric changes such as wind, cold, heat, dampness and dryness are all considered evil qi, if they happen in an untimely fashion or in excess.
In this context, disease is defined as a process of struggle between the genuine qi and evil qi. Between these two kinds of qi, emphasis is often put on the former, as is stated in The Medical Classic of the Yellow Emperor: "If genuine qi prevails, attack of evil qi never avails."
Regarding the classification of genuine qi and the clinical significance of the various part of genuine qi.