This article examines how Marguerite Porete defended her ideas in her mystical treatise The Mirror of Simple Souls, which along with its author was condemned as heretical in 1310. Most scholarship has focussed on the final sixteen chapters of the Mirror as evidence of Marguerite's self-defence. This article shows that Marguerite was concerned with defending her ideas throughout the course of composing the Mirror, and not merely while writing the final chapters. Focussing on two key concepts in the Mirror which were singled out at her trial in Paris, it shows how Marguerite repeatedly presented these concepts in ways which were meant to shield them from criticism. The article then examines the reactions of two later readers of the Mirror to these defences, exploring their successes and failures and the vastly different ways in which they could be interpreted.