This paper reconstructs the chronology, content, and consequences of al-Ghazālī's first crisis of knowledge for his epistemological doctrine. In so doing, we explore his teaching on: 1) sense perceptions, 2) axiomatic cognitions, and 3) the knowledge-claims which are made on the basis of each source. We also explain why his skepticism emerged, what its exact dimensions were, how his skeptical interlude ended, and what the restoration of certitude about rational knowledge entailed for him in the approximate period between 461/1068-1069 and 488/1094-1095. In answering these questions, we emphasize and clarify the role that popular Sufism, especially in terms of its doctrine of esoteric knowledge or ma'rifa, played in the formation of al-Ghazālī's essential perspective on rational knowledge.