"This dissertation examines the thought of Welsh theologian and magician, Thomas Vaughan (Eugenius Philalethes). I argue that Vaughan played an important role in the ongoing theological, scientific, and magico-alchemical conversations of seventeenth-century Britain. While literary scholars have considered Vaughan in relation to his brother Henry, and historians of science have started to review Vaughan's alchemical work, to my knowledge, Thomas has never been considered as a theologian or magician. Through close readings of Vaughan's published writings, analyses of their public reception, and explorations of the writers who influenced Vaughan, I seek to make a case for Vaughan as a "theomagus," or Christian magician. Vaughan was involved in the universal reform movement of Samuel Hartlib and allied himself with a magical branch of reform associated with the late fifteenth-century humanist Marsilio Ficino and sixteenth-century magician Heinrich Cornelius Agrippa. Vaughan sought to restore peace and religious unity through the prisca theologia (or 'original theology'), a primordial wisdom inherent in Creation, but lost to humanity through the Fall of Adam and subsequent ages of sin."