Benjamin Isaac. Such proto-racism and ethnocentrism must be looked at within context, because a modern understanding of racism based on hereditary inferiority (with modern racism based on eugenics and scientific racism) was not yet developed and it is unclear whether Aristotle believed the natural inferiority of Barbarians was caused by environment and climate (like many of his contemporaries) or by birth. Historian Dante A. Puzzo, in his discussion of Aristotle, racism, and the ancient world writes that: Racism rests on two basic assumptions: that a correlation exists between physical characteristics and moral qualities; that mankind is divisible into superior and inferior stocks. Racism, thus defined, is a modern conception, for prior to the XVIth century there was virtually nothing in the life and thought of the West that can be described as racist. To prevent misunderstanding a clear distinction must be made between racism and ethnocentrism ... The Ancient Hebrews, in referring to all who were not Hebrews as Gentiles, were indulging in ethnocentrism, not in racism. ... So it was with the Hellenes who denominated all non-Hellenes—whether the wild Scythians or the Egyptians whom they acknowledged as their mentors in the arts of civilization—Barbarians, the term denoting that which was strange or foreign. Medieval Arab writers Bernard Lewis has also cited historians and geographers of the Middle East and North Africa region, including Al-Muqaddasi, Al-Jahiz, Al-Masudi, Abu Rayhan Biruni, Nasir al-Din al-Tusi, and Ibn Qutaybah. Though the Qur'an expresses no racial prejudice, Lewis argues that ethnocentric prejudice later developed among Arabs, for a variety of reasons: their extensive conquests and slave trade; the influence of Aristotelian ideas regarding slavery, which some Muslim philosophers directed towards Zanj (Bantu) and Turkic peoples; and the influence of Judeo-Christian ideas regarding divisions among humankind. By the eighth century, anti-black prejudice among Arabs resulted in discrimination. A number of medieval Arabic authors argued against this prejudice, urging respect for all black people and especially Ethiopians. By the 14th century, a significant number of slaves came from sub-Saharan Africa; Lewis argues that this led to the likes of Egyptian historian Al-Abshibi (1388–1446) writing that "[i]t is said that when the [black] slave is sated, he fornicates, when he is hungry, he steals." According to Lewis, the 14th-century Tunisian scholar Ibn Khaldun also wrote: ...beyond [known peoples of black West Africa] to the south there is no civilization in the proper sense. There are only humans