The usage of ‘entropy’ in philological studies dates back to d’A.S. Avalle and is part of a group of concepts borrowed from nineteenth-century physics in the framework of the methodological renewal promoted by Italian neo-Lachmannism. Entropy helps describe (and individuate, to a certain extent) phenomena of textual innovation that cannot be classified by stemmatic means. After defining the concept and its past and present uses, this paper shows how an in-depth study of some major entropic trends can considerably reduce subjectivity when assessing polygenesis and monogenesis (esp. during the recension), and in distinguishing substantials and accidentals (prior to the preparation of the critical text). Our examples are taken from two Arthurian prose romances, the Roman de Méliadus and the Prose Lancelot, and compared with the varia lectio from a corpus of Middle English copies recently analysed by D. Wakelin.