On the Nature of Man
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On the Nature of Man is a work in the Hippocratic Corpus. On the Nature of Man is attributed to Polybus, the son in law and disciple of Hippocrates, through a testimony from Aristotle's History of Animals.However as with the many other works of the Hippocratic Corpus, the authorship is regarded as dubious in origin.
On the Nature of Man attempts to explain the human body in its anatomy and composition. The content is based on observation and defended by logical explanations of the causes of each disease in order to readily meet outside criticism. It places emphasis on disease not being of divine origin, but rather an imbalance of the four humors (collection of blood, phlegm, black bile, and yellow bile) in the body.
On the Nature of Man shares the general Hippocratic interest in humorism and in such treatments as bloodletting. Bloodletting is the procedure performed in order to regulate the patient's four humors:
"Furthermore, one must know that diseases due to repletion are cured by evacuation, and those due to evacuation are cured by repletion; those due to exercise are cured by rest, and those due to idleness are cured by exercise."
On the Nature of Man gives first hand accounts and explanations of individual medical cases. For example, dysentery and nosebleeds occur in the spring and summer because this is when the blood is at its hottest. Hippocrates concludes that the degree of damage a given disease can do to a person depends on its nature. The most serious of illnesses are those that affect the strongest part of the body. If the strongest part of the body is affected, then the weak parts are easily affected and may cause death. However, if a disease starts in a weak area of the body, often it is curable. 
- Lindberg, David C. (2007). The Beginnings of Western Science. Chicago: The University of Chicago Press.
- Hippocrates; Translated by W.H.S. Jones. Heracleitus On the Universe Vol. IV. New York: G.P Putnam's Sons.
- Gillispie, Charles Coulston (1972). Dictionary of Scientific Biography VI. New York: Charles Scribner's Sons. pp. 419–427.