Black bile is the strangest of the four. Blood, yellow bile and phlegm each correspond to something a physician could point at; black bile does not. It was assigned a seat in the spleen and held to be cold and dry, which places it with earth among the elements, with autumn among the seasons, and with maturity among the ages of life, but no fluid answering the description was ever produced.
The doctrine survived the difficulty for two millennia because it explained too much to be given up. An excess of a cold dry humour accounted for the settled, heavy, fearful and sorrowful states that had no obvious external cause, and it did so inside the same framework that explained fevers and catarrhs, which is a considerable economy. Darkened blood and the residues of certain diseases were pressed into service as evidence, but the humour was in practice inferred from the condition it had been invoked to explain.
That circularity is worth stating plainly, because it is the clearest single case in the scheme of a category kept for its usefulness rather than for its evidence. The temperament built on it, however, was described with real observational care, and the accounts the older writers give of the melancholic habit, its wakefulness, its fixity of attention and its dread without an object, remain recognisable to anyone reading them now.