Too-frequent ejaculation was thought to weaken men. Greek medical theories based on the classical elements and humors recommended limiting the production of semen by means of cooling, drying, and astringent therapies, including cold baths and the avoidance of flatulence-causing foods. In the 2nd century AD, the medical writer Galen explains semen as a concoction of blood (conceived of as a humor) and "pneuma" (the "vital air" required by organs to function) formed within the man's coiled spermatic vessels, with the humor turning white through heat as it enters into the testicles. In his treatise "On Semen", Galen warns that immoderate sexual activity results in a loss of "pneuma" and hence vitality: