This article presents the results of a qualitative case study on the diaries of Etty Hillesum. The focus of this investigation is on the way her inner development leads to increased sensitivity. This research revealed that Etty’s inner development is characterized by a transformation from great opacity to transparency in three phases. Whereas she is at first lost in inner chaos, she shows an impressive serenity and inner peace at the end of her writings. It is argued that in each of the three phases, Etty adapts her strategies to the new challenges she has to face in life. The main processes involved here are "mental cleansing," "mental slimming," and "mental emptying." Going through these processes, she codevelops the sensitivity as well as the firmness she needs to face reality in all its aspects.