When debating the need for prophets, Muslim theologians frequently cited an objection from a group called the Barāhima – either a prophet conveys what is in accordance with reason, so they would be superfluous, or a prophet conveys what is contrary to reason, so they would be rejected. The Barāhima did not recognise prophecy or revelation, because they claimed that reason alone could guide them on the right path. But who were these Barāhima exactly? Were they Brahmans, as their title would suggest? And how did they become associated with this highly incisive objection to prophecy? This book traces the genealogy of the Barāhima and explores their profound impact on the evolution of Islamic theology. It also charts the pivotal role that the Kitāb al-Zumurrud played in disseminating the Barāhima’s critiques and in facilitating an epistemological turn in the wider discourse on prophecy (nubuwwa). When faced with the Barāhima, theologians were not only pressed to explain why rational agents required the input of revelation, but to also identify an epistemic gap that only a prophet could fill. A debate about whether humans required prophets thus evolved into a debate about what humans could and could not know by their own means.