This paper presents a reflection on the reasons which motivate the evaluation of restorative justice practices. In a social and institutional domain crowded with believes and symbols it debates what this need for understanding means. With the help of simple questions: who evaluates, for what reasons, what is evaluated and how, it proposes to researchers a small exercise in scientific hygiene useful for both improving their necessary distance and for understanding better the difficulties of evaluative reasoning within the field of restorative justice. (author's abstract)