Many social changes took place under Islam between 610 and 661, including the period of Muhammad's mission and the rule of his four immediate successors who established the Rashidun Caliphate. Historians generally agree that Islamic social reforms in areas such as social security, family structure, slavery and the rights of women and ethnic minorities improved on what was present in existing Arab society. For example, according to Lewis, Islam "from the first denounced aristocratic privilege, rejected hierarchy, and adopted a formula of the career open to the talents."