The human body contains blood, phlegm, yellow bile, and black bile.  These are the things that make up its constitution and cause its pain and health.  <B>Health is primarily that state in which these constituent substances are in the correct proportion to each other, both in strength and quantity, and are well-mixed.</B> Pain occurs when one of the substances presents either a deficiency or an excess, or is separated in the body and not mixed with others.

Hippocratic Writings, page 262