Varying conceptions of intellect ('aql) in Shī'a Islam, and in particular Imāmī Shī'ism, have been perennial and contested themes, affecting all aspects of Shī'ite religious and intellectual life, from the theological to the jurisprudential, the philosophical to the mystical. A main hadith source frequently referred to in this debate is the famous Usūl al-kāfī, particularly the first chapter: "The Book of Intellect and Ignorance." This paper explores a basic conception of 'aql as understood from the traditions in the Usūl al-kāfī. Having defined 'aql from the Shī'ī hadith-reports, I will show that the larger debate about definitions of intellect and the varying positions it plays in Imāmī thought and spirituality reflect the diversity of possible readings of this chapter in the Usūl al-kāfī. The Safavid period saw a surge of commentaries on this text and led to various intellectual debates within the world of Imāmī Shī'ism, between the text-focused Akhbaris, the more rational Usulis, and the theosophers and mystics. Indeed, Usūl al-kāfī, and this particular chapter on the Intellect, can not only be seen as the locus of much debate, but also a necessary source of authority to which these various intellectuals turn, presenting a complex relationship between the reader and the text, a relationship where the intellect reigns supreme.