but rather on the personal assumptions of Muslim historians about the nature of Islam, Hinduism and Buddhism in the northwest Indian subcontinent. Richard Eaton, a professor of history at Berkley, asserts that the presumption of a rigid Hindu caste system and the oppression of lower castes in pre-Islamic era in India is the cause of "mass conversion to Islam" during the medieval era. This claim has the problem that "no evidence can be found in support of the theory, and it is profoundly illogical". Peter Jackson, a professor of Medieval History and Muslim India, writes that the speculative hypotheses about the caste system in Hindu states during the medieval Delhi Sultanate period (~1200 to 1500), and the existence of a caste system, as being responsible for Hindu weakness in resisting the plunder by Islamic armies, is appealing at first sight, but "they do not withstand closer scrutiny and historical evidence". Jackson states that, contrary to the theoretical model of caste where only Kshatriyas could be warriors and soldiers, historical evidence confirms that Hindu warriors and soldiers during the medieval era included members other castes such as Vaishyas and Shudras. "Further," writes Jackson, "there is no evidence that there ever was a widespread conversion to Islam at the turn of twelfth century" by Hindus of lower caste. Jamal Malik, a professor of Islamic studies, extends this observation further, and states that "at no time in history did Hindus of low caste convert en masse to Islam". Jamal Malik states that caste as a social stratification is a well-studied Indian system, yet evidence also suggests that hierarchical concepts, class consciousness and social stratification had already occurred in Islam before Islam arrived in India. The concept of caste, or 'qaum' in Islamic literature, is mentioned by a few Islamic historians of medieval India, states Malik, but these mentions relate to the fragmentation of the Muslim society in India. Zia al-Din al-Barani of Delhi Sultanate in his Fatawa-ye Jahandari and Abu al-Fadl from Akbar's court of Mughal Empire are the few Islamic court historians who mention caste. Zia al-Din al-Barani's discussion, however, is not about non-Muslim castes, rather a declaration of the supremacy of Ashraf caste over Ardhal caste among the Muslims, justifying it in Quranic text, with "aristocratic birth and superior genealogy being the most important traits of a human". Irfan Habib, an Indian historian, states that Abu al-Fazl's Ain-i Akbari provides a