The Four Humors
The most historically prominent theoretical scheme of the Coan writers was the doctrine of the four humors of the body: blood, phlegm, black bile, and yellow bile (or sometimes serum).
Health was defined as the balance of the four humors. Disease was defined as the imbalance of the humors. When imbalance occurred, then the physician might intervene by making a correction to bring the body back into balance. For example, if the individual were too full of phlegm (making her phlegmatic or lethargic), then the phlegm must be countered.
Citrus fruit was thought to be a counter-acting agent. Thus, if one feels lethargic, increasing one’s citrus intake will re-create balance. The treatment is, in fact, generally effective. Moderns might describe the therapy differently by ascribing the effect to vitamin-C, phosphorus, and natural sugar.
This example illustrates the scope of the Hippocratic physician in this context: something like a cross between the modern roles of an herbalist dietician and a personal trainer. Nonetheless, the cures that were dictated by the four humor theory seemed to work well enough for this theory to extend to the nineteenth century (in various guises).