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In the theory of the 'four humors' there were four special fluids associated with the four basic elements of nature (earth, air, fire, water), that were thought to permeate the body and influence its health. This was an important idea in Western medicine from the time of Hippocrates (400 B.C.E.) until Renaissance times. An imbalance in the distribution of these fluids was thought to affect each individual's personality. The word itself for 'humor' comes from the Greek word 
$\chi\upsilon\mu\omicron\varsigma$ 
(CHUMOS), literally 'sap' or 'fluid'.

In the course of Hippocrates' studies he noticed that blood removed from the body separates into four parts: the clear red, a yellowish liquid that rises to the top, the dark liquid that settles to the bottom, and whitish fluid. He and his students, especially his son-in-law, Polybus, took these observations and developed a theory of medicine that was to hold sway in the West and in the Islamic world for thousands of years\EmDash{}a theory further expounded upon by Galen (129\EnDash{}200): that physical and mental health was a matter of a good balance of four liquids (``humors''), all believed to be produced in the liver, but which are found in the veins and are associated with various organs of the body. 

Each humor corresponded to an element, a personality type, a quality, and an organ in the body: 

\begin{center}
\scriptsize
\begin{tabular}{|l|l|l|l|l|}
\hline
Humor				& Element		& Temperament													& Quality 			& Organ\\
\hline
blood 			& air				& Sanguine (courageous, amorous)			& hot \& moist	& liver\\
black bile	& earth			& Melancholic (despondent, sleepless)	& cold \& dry		& spleen\\
yellow bile	& fire			& Choleric (easily angered)						& hot \& dry		& gall bladder\\
phlegm			& water			& Phlegmatic (calm, unemotional)			& cold \& moist	& brain/lungs\\
\hline
\end{tabular}
\normalsize
\end{center}
