The literary sources in Arabic dealing with pre-Islamic Arabia are copious, but rarely give direct answers to questions which are of interest to modern research. Arabian society was tribal and included nomadic, semi-nomadic and settled populations. The biography of Muhammad provides further evidence of the cooperation between the nomadic and settled populations. The Arab idol worshippers were polytheists, but they also believed in a High God called Allah whose house was in the Kaba and who had supremacy over their tribal deities. In Medina, which was in many ways different from Mecca, idols were associated with various levels of the tribal organization. The Byzantines and Sasanians conducted their Arabian affairs through their respective Arab buffer kingdoms, Ghassan and al-Hira. Caravan trade was often behind the cooperation between certain nomadic tribes and the Sasanians. In addition to trade, the entrepreneurial Qurashis invested in agriculture. Since conditions in Mecca itself were uninviting for agriculture, they looked for opportunities elsewhere.