Fakhr al-Dīn al-Rāzī’s (d. 606/1210) Kitāb al-Sirr al-Maktūm (Book of the Hidden Secret) was an immensely popular work on magic comprising sections on astrology, talismans, planetary invocations, incantations, and other occult operations. Unlike authors of other, similar works, al-Rāzī never claims to be a practitioner of the magic that he describes in this text. He instead positions himself as a compiler and a theorist. So how does he know that the operations he includes are effective? What is the role of experimentation and experience (tajriba) in his work? In this paper, I first summarize al-Rāzī’s theory about the sources for knowledge about magical operations as outlined in al-Sirr al-Maktūm. For al-Rāzī, most of this knowledge is rooted in inspiration (ilhām) or revelation (waḥy), but experience (tajriba) still plays an important role in verifying this material. I then examine how al-Rāzī, as a non-practitioner, establishes his authority and positions himself as a compiler and editor. Al-Rāzī’s authorial voice is compared to that of two author-practitioners of similar magical compendia. The first is Abū al-Faḍl Muḥammad al-Ṭabasī, whose Kitāb al-Shāmil fī al-baḥr al-kāmil was a major source for al-Rāzī; the second is Sirāj al-Dīn al-Sakkākī, a younger contemporary of al-Rāzī who cites from al-Sirr al-Maktūm in his own (similarly titled) compendium Kitāb al-Shāmil fī baḥr al-kāmil. Finally, taking three spells as examples, I analyze the role of experimentation, observation, and trial and error in instructions for performing magic spells. I argue that first-hand experience is consistently valued and used as a mark of legitimacy and validity throughout al-Sirr al-Maktūm, despite the fact that it is largely a compilation from other written sources, one that sees inspiration to be the main source for knowledge about magical practice.