Abstract
Probabilism is a Scholastic discussion of conscience beginning in the sixteenth century and lasting over four hundred years. To tackle historical issues in normative ethics, the participants had to work out a general "metaethical" theory relating epistemic and deontic logics, the logics of knowledge and virtue. May I act if I am unsure that I may? How, when I am in doubt, can I acquire the critical mass of rationality that virtue demands? The normative aspect of the controversy has been studied in its historical setting, but its interesting and original metaethics has been largely ignored outside the Scholastic circle itself. The discussion is obviously relevant, even practical, today; in fact, any treatment of conscience and relativism already includes the material of probabilist analyses.