I wonder if it doesn't have something to do with Avicenna's binary oppositions in relation to Galen's four temperaments (or humors): hot, cold, wet, moist. Choleric was dry and hot, and melancholic was dry and cold. (A note to those driven mad by the modern use of a word like decimate, seem meekly content with the change of (etymological) meaning of humor from liquidity to tempermentality.)

Ceci n'est pas un seing.