Early medical science believed that there were four humors which combined to form the chyle (q.v.).
These were divided along two physical axes: wet-dry and warm-cold. The four humors were:
The idea balanced against that of four elements (earth, air, fire and water) and the four seasons as below:
A healthy body was thought to exhibit a mix of all four humors, with the relative strengths of each
giving rise to personality and behavior. When one humor predominated, it was thought, physical or mental illness resulted.
Treatment of specific illnesses could be attempted by applying the polar opposite treatment. A fever was warm and dry, and thus an excess of choler. Applying a cold and wet treatment such as a bath or cool towel was proscribed. A cold and its resulting phlegm were treated with warmth and dryness. Simplistic though this may seem now, it was one of the first steps towards treatment based on medical diagnosis.
As BaronCarlos' writeup points out, we still use English words today for specific states of mind that derive from these ideas: specifically Melancholy, Sanguine, Choleric and Phlegmatic.
It is amusing to note that many of the "personality" analysis models used by corporate training consultants use a thinly disguised version of this same idea. If you've been through an analysis that tells you you're an "Expressive Analytic" or a "Blue with Red" or a "Dominant Inquirer" or whatever, you're seeing this same four-quadrant analysis with modern terms. They might as well tell you you're full of yellow bile and have done with it.