This article seeks to resituate critical discussions about logic in the Old French Grail romances and Thomas Malory’s Tale of the Sankgreal. Where previous scholarship has emphasised the mystical elements of the Old French Grail narratives to suggest alternate meanings for the Grail itself, this article reads the Grail miracles as structuring devices that reflect classical theories of dialectic and demonstrative argumentation. Through examining one example from Chrétien de Troyes’ Perceval, the Didot-Perceval, The Vulgate Cycle Queste del Saint Graal, and Thomas Malory’s Tale of the Sankgreal, this article also highlights fundamental similarities between the logical systems underlying each Grail narrative that are not restricted by language or date of composition. Thus, the article depicts Malory not just as consciously drawing upon the Vulgate Queste del Saint Graal, but also as unconsciously inheriting elements from each of his Old French predecessors.