In 1377 the Islamic scholar Ibn Khaldun, in his "Muqaddima" (or Prolegomena), wrote that when nomadic tribes become united by "asabiyya"—Arabic for "group feeling", "social solidarity", or "clannism"—their superior cohesion and military prowess puts urban dwellers at their mercy. Inspired often by religion, they conquer the towns and create new regimes. But within a few generations, writes Ibn Khaldun, the victorious tribesmen lose their "asabiyya" and become corrupted by luxury, extravagance, and leisure. The ruler, who can no longer rely on fierce warriors for his defense, will have to raise extortionate taxes to pay for other sorts of soldiers, and this in turn may lead to further problems that result in the eventual downfall of his dynasty or state.