The concepts of Yin and Yang and the Five Agents provided the intellectual framework of much of Chinese scientific thinking especially in fields like biology and medicine the organs of the body were seen to be interrelated in the same sorts of ways as other natural phenomena, and best understood by looking for correlations and correspondences. Illness was seen as a disturbance in the balance of Yin and Yang or the Five Agents caused by emotions, heat or cold, or other influences. Therapy thus depended on accurate diagnosis of the source of the imbalance.

The earliest surviving medical texts are fragments of manuscript from early Han tombs. Besides general theory, these texts cover drugs, gymnastics, minor surgery, and magic spells. The text which was to become the main source of medical theory also apparently dates from the Han. It is the Yellow Emperor's Classic of Medicine, supposed to have been written during the third millennium BCE by the mythical Yellow Emperor. A small extract is given below.

The Yellow Emperor said ''The principle of Yin and Yang is the foundation of the entire universe. It underlies everything in creation. It brings about the development of parenthood; it is the root and source of life and death it is found with the temples of the gods In order to treat and cure diseases one must search for their origins.
"Heaven was created by the concentration of Yang, the force of light; earth was created by the concentration of Yin, the forces of darkness. Yang stands for peace and serenity; Yin stands for confusion and turmoil. Yang stands for destruction; Yin stands for conservation. Yang brings about disintegration; Yin gives shape to things.
The pure and lucid element of light is manifested in the upper artifices and the turbid element of darkness is manifested in the lower orifices. Yang, the element of light, originates in the pores. Yin, the element of darkness, moves within the five viscera. Yang the lucid force of light truly is represented by the four extremities- and Yin the turbid force of darkness stores the power of the six treasures of nature. Water is an embodiment of Yin as fire is an embodiment of Yang. Yang creates the air, while Yin creates the senses, which belong to the physical body when the physical body dies; the spirit is restored to the air, its natural environment. The spirit receives its nourishment through the air, and the body receives its nourishment through the senses

"If Yang is overly powerful, then Yin may be too weak. If Yin is particularly strong, then Yang is apt to be defective if the male force is overwhelming, then there will be excessive heat. If the female force is overwhelming, then there will be excessive cold. Exposure to repeated and severe heat will induce chills. Cold injures the body while heat injures the spirit when the spirit is hurt, severe pain will ensue. When the body is hurt, there will be swelling. Thus, when severe pain occurs first and swelling comes on later, one may infer that a disharmony in the spirit has done harm to the body. Likewise, when swelling appears first and severe pain is felt later on, one can say that a dysfunction in the body has injured the spirit.

"Nature has four seasons and five elements. To grant long life, these seasons and elements must store up the power of creation in cold, heat, dryness, moisture, and wind. Man has five viscera in which these five climates are transformed into joy, anger, sympathy, grief, and fear. The emotions of joy and anger are injurious to the spirit just as cold and heat are injurious to the body. Violent anger depletes Yin; violent joy depletes Yang. When rebellious emotions rise to Heaven the pulse expires and leaves the body when joy and anger are without moderation, then cold and heat exceed all measure, and life is no longer secure. Yin and Yang should be respected to an equal extent.
The Yellow Emperor asked, "Is there any alternative to the law of Yin and Yang?" Qi Bo answered: "When Yang is the stronger, the body is hot, the pores are closed, and people begin to pant; they become boisterous and coarse and do not perspire. They become feverish, their mouths are dry and sore, their stomachs feel tight, and they die of constipation. When Yang is the stronger, people can endure winter but not summer. When Yin is the stronger, the body is cold and covered with perspiration. People realize they are ill; they tremble and feel chilly. When they feel chilled, their spirits become rebellious. Their stomachs can no long digest food and they die. When Yin is stronger, people can endure summer but not winter. Thus Yin and Yang alternate. Their ebbs and surges vary, and so does the character of the diseases."
The Yellow Emperor asked, "Can anything be done to harmonize and adjust these two principles of nature?" Qi Bo answered: "If one has the ability to know the seven injuries and the eight advantages, one can bring the two principles into harmony. If one does not know how to use this knowledge, his life will be doomed to early decay. By the age of forty the Yin force in the body has been reduced to one-half of its natural vigor and an individual's youthful prowess has deteriorated. By the age of fifty the body has grown heavy. The ears no longer hear well. The eyes no longer see clearly and by the age of sixty the life producing power of Yin has declined to a very low level. Impotence sets in and the nine orifices no longer benefit each other.
Those who seek wisdom beyond the natural limits will retain good hearing and clear vision. Their bodies will remain light and strong. Although they grow old in years, they will stay able-bodied and vigorous and be capable of governing to great advantage. For this reason the ancient sages did not rush into the affairs of the world. In their pleasures and joys they were dignified and tranquil. They did what they thought best and did not bend their will or ambition to the achievement of empty ends Thus their allotted span of life was without limit, like that of Heaven and earth. This is the way the ancient sages controlled and conducted themselves.
"By observing I learn about others, and their diseases become apparent to me. By observing the external symptoms, I gather knowledge about the internal diseases One should watch for things out of the ordinary One should observe minute and trifling things and treat them as if the were big and important. When they are treated the danger they pose will be dissipated. Experts in examining patients judge their general appearance; they feel their pulse and determine whether it is Yin or Yang that causes the disease.... To determine whether Yin or Yang predominates, one must be able to distinguish a light pulse of low tension from a hard. Pounding one with a disease of Yang, Yin predominates with a disease of Yin, Yang predominates when one is filled with vigor and strength, Yin and Yang are in harmony.