I believe he expanded on Hippocrates' theory of humors by associating a specific human temperament (passion, kindness, etc.) to each humor.
Galen expanded Hippocrates' theory of humors by linking each of the four humors to specific emotional and physical characteristics. The humors are blood (sanguine), yellow bile (choleric), black bile (melancholic), and phlegm (phlegmatic), each associated with different seasons and elements. This theory significantly impacted medical thought during the Middle Ages and beyond.
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