This article analyzes the involvement of Thomas of Bailly (d. 1328), secular master of theology and penitentiary of the diocese of Paris, in the examination of extracts taken from Marguerite Porete's "Mirror of Simple Souls." To date, no one has examined the precise theological formulations that led Thomas and twenty other theologians to recommend that the Mirror be consigned to the flames. By examining Thomas's career and writings, this article unearths the theological scaffolding that the extracts brushed aside, enriching our understanding of one of the most famous heresy trials of the Middle Ages.