This work provides a careful historical analysis of the methods and contents of the 'De Trinitate' of Boethius and the 'Expositio' of Aquinas. It is also the first comprehensive philosophical and theological analysis of Aquinas' 'Expositio' to be based on the modern critical edition of the Latin text and in the light of mid- and late-20th-century advances in thomistic scholarship. In this study the author locates the reconstruction of Boethius undertaken by Aquinas, and radically documents the dialectical themes of 'agnosia' and 'remotion' in this work. Such documentation aims to provide a higher level of understanding of the structure of the 'cu Expositio' than is possible with mid-20th-century approaches which have emphasized participationist, analogical and transcendental thematics.