Gold and silver have been found abundantly in the tombs, not only as jewelry, but as vessels, weapons, ornaments, even as tools. Rich and poor were stratified into many classes and gradations; slavery was highly developed, and property rights were already sacred.32 Between the rich and the poor a middle class took form, composed of small-business men, scholars, physicians and priests. Medicine flourished, and had a specific for every disease; but it was still bound up with theology, and admitted that sickness, being due to possession by evil spirits, could never be cured without the exorcising of these demons. A calendar of uncertain age and origin divided the year into lunar months, adding a month every three or four years to reconcile the calendar with the seasons and the sun. Each city gave its own names to the months.