This study shows how Grosseteste’s Commentary on the Posterior Analytics adopts a peculiar criterion to structure arguments developed by Aristotle about the science of demonstration. Grosseteste places his commentary in the framework of the ratio disserendi by Cicero and Boethius; yet, the arguments he develops are summarised in conclusiones. After reviewing past and recent scholarship on the meaning of these ‘conclusions’ and their relationship to Aristotle’s arguments, the study proposes to consider them as an application of Euclid’s geometrical method.