Sanguine

The humoral scheme sets four fluids in the body and explains health as their balance and illness as an excess or a deficiency of one of them. It appears in its familiar four-part form in a Hippocratic treatise on the nature of man written in the late fifth century BCE, and it was argued for, systematised and made dominant by Galen in the second century CE. Blood is the humour of this temperament, and in the physiology that went with the scheme it was made in the liver out of digested food and carried outward through the veins.

Each humour was assigned a pair of the primary qualities, hot or cold together with moist or dry, and blood took hot and moist. That pairing fixes the rest of the correspondences: air among the elements, spring among the seasons, and childhood or early youth among the ages of life, each of which is likewise hot and moist. The same four-part structure repeats across all of them, and the neatness of that repetition is a large part of why the scheme was found satisfying for so long.

Bloodletting, the most persistent therapeutic practice in the Western tradition, follows directly from it. If illness is an excess and blood is the humour most easily removed, then drawing blood is the obvious correction, and physicians drew it for conditions that have nothing to do with any modern account of the blood for the better part of two thousand years.

A sanguine person, in the strict sense the doctrine intended, was not a cheerful person but a person in whom blood predominated. The physical signs came first: a ruddy complexion, a full habit of body, a strong pulse, warm skin. The character was held to follow from the physiology rather than to stand on its own, and a physician who judged a patient sanguine was making a claim about the body before making one about the disposition.

Astrological medicine took the scheme up and attached it to the planets, giving the sanguine complexion to Jupiter, whose traditional qualities of heat and moisture match it exactly. Practitioners such as William Lilly, writing in seventeenth century England, judged a person's complexion from the birth chart and used the judgement in medical and ordinary questions alike, so the temperament became something read off a birth time as well as observed at a bedside.

What survives in English is the character and not the physiology. Sanguine now means optimistic and confident about an outcome, with no implication about anybody's blood, and it is the only one of the four whose modern sense has drifted almost wholly to the positive. Cheerfulness was never the whole of it for the older writers, who paired the complexion with a tendency to be changeable, to promise more than was delivered and to lose interest before a thing was finished. The narrowing to plain good humour is a modern simplification of a portrait that had a shadow in it.

Sources Christian AstrologyWilliam Lilly with On the Affected Parts / On My Own Opinions (Galenic corpus, reference synthesis)Galen; reference synthesis by Wikipedia contributors