N i AHUATL Indians living in the southern part of the Valley of Mexico have integrated the European hot-cold complex into a native system of complementary opposites controlling the ordering of the universe. The European hot-cold complex, commonly called the Hippocratic system, was introduced into America by the Spanish invaders of the sixteenth century. Originally a part of Greek and Roman medicine, the Hippocratic system rested on the theory that all substances were made up of the elements of earth, air, fire, and water in varying proportions, and consequently they possessed the qualities of cold, dry, hot, and wet in varying degrees. The qualities were opposed: cold was the opposite of hot, and wet the opposite of dry. Hence a substance could be described in terms of only two qualities. A statement that it was hot in the second degree and dry in the third degree gave its position in terms of both oppositions. This theory had its application in medicine through the belief that a healthy body is in a state of equilibrium in terms of each of these classic oppositions; in sickness, the balance is lost and the body becomes too cold or too hot, too wet or too dry. Part of the treatment involved restoring the balance of the body by the prescription of foods and medicines of qualities opposite to those produced in the human body by sickness. Thus hot foods and medicines were indicated for an illness producing coldness. Elaborate knowledge of the qualities of foods and herbs was required of physicians and was a matter of public knowledge as well, at least as far as the cure of simple illnesses by home remedies was involved. Hot, cold, wet, and dry in this system were abstract qualities and had nothing necessarily to do with the actual temperature or wetness of the subject matter. Liquids might be classified as dry and a hot soup might be classified as cold if it were made from an herb to which the abstract quality of coldness was attributed. The Hippocratic system was taken up by Renaissance physicians and introduced into America, where it remained dominant in Spanish medical circles until about the middle of the eighteenth century, but the gap between the universities and the people was so great that it continued to flourish vigorously long after it ceased to be respectable medical doctrine and it is to this day the basis of folk curing. The hot-cold system is also the basis of the popular view of the world in general in some cases. During its long history in the New World, the Hippocratic system has been greatly simplified. Modern versions of it generally lack the wet-dry opposition, and, in many cases, the practice of ranking the qualities present by degrees (originally four degrees for each quality). There may be terms of locally recognized intermediate stages within the hot-cold opposition, however. Very interesting changes have taken place in the details of the system, and it now differs, both in characteristics and in