“The definition of Sadness. Sadness is a disagreeable languor in which consists the discomfort and unrest which the soul receives from evil, or from the defect which the impressions of the brain set before it as pertaining to it. And there also is an intellectual sadness which is not passion, but which hardly ever fails to be accompanied by it.
The movement of the blood and spirits in Sadness. In sadness, the openings of the heart are much contracted by the small nerve which surrounds them, and the blood of the veins is in nowise agitated, which brings it to pass that very little of it goes toward the heart and yet the passages by which the juice of the food flows from the stomach and the intestines towards the liver remain open, which causes the appetite not to diminish at all, excepting when hatred, which is often united to sadness, closes them.”
Descartes knew some stuff.