 Medieval and Renaissance philosophers used probability-related vocabulary; but they did not analyze the underlying concepts in depth, at least in comparison with the thoroughness of their investigations into the notions of knowledge or science (scientia). Probability-related terms were usually introduced in passing, and the most differentiated treatments extended to a folio page at best. Nevertheless, issues of probability were not peripheral to medieval thought. Probability-related terms and concepts played an enormous role in the regulation of everyday conduct and in many arts and sciences. Rhetoric and dialectic were by definition concerned with persuasion, credibility, and probability. Yet medieval thinkers were also highly aware that many claims in jurisprudence, theology, and medicine could not be derived with certainty from indubitable premises. Hence, arguments in these fields had to rely more often than not on probable reasoning. The notorious disagreements between scholastic academics in their fields of inquiry, nourished and represented by the practice of scholastic disputation, further increased the demand for probable reasoning. Wherever disagreement was licit and impossible to extirpate by argument, the claims of all sides counted as merely probable. Since medieval theologians and lawyers were no more in agreement concerning the ascription of sins (or of moral actions) than on other matters, the guidance of human conduct by confessors was also a matter of probability. In this domain, the regulation of decisions of conscience relied on a systematic framework of probable reasoning and decision in conditions of uncertainty. In short, for a satisfactory understanding of the medieval intellectual edifice, inspection of the uses of probability seems indispensable.