 Probability-related terminology played an important role in medieval and Renaissance philosophy. Terms such as ‘probable’ (probabilis), ‘credible’ (credibilis) or ‘truth-like’ (verisimilis) were used to assess philosophical claims, qualify uncertain conclusions, gauge the force of arguments and temper academic disagreement. Beyond that, they had a significant impact on the regulation of legal proceedings, moral action and everyday life. The probability-related terminology of the Middle Ages descended from ancient sources such as Aristotle, Cicero and Boethius. There is no precedent, however, for many medieval ways of connecting these terms to rules and principles for legal and practical decision-making.