 MacLein states the caste and conversion theories about Indian society during the Islamic era are not based on historical evidence or verifiable sources, but personal assumptions of Muslim historians about the nature of Islam, Hinduism and Buddhism in northwest Indian subcontinent.Richard Eaton, a professor of History, states that the presumption of a rigid Hindu caste system and oppression of lower castes in pre-Islamic era in India, and it being the cause of "mass conversion to Islam" during the medieval era suffers from 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 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".