Judges became increasingly professionalized in Western Europe in the thirteenth century. Rather than being simply people of good character and high standing in their communities, they were expected to have extensive legal expertise. Yet the requirements of their professions raised specific ethical and epistemological problems. In this presentation, I will explore some of these problems in relation to the frequently debated question whether judges should pronounce a judgment according to what is alleged and proved in court or according to what they know to be the case. My focus will be on the neglected arguments that philosophers such as Thomas Aquinas, Richard of Middleton, Gerard of Odo, Thomas de Vio Cardinal Cajetan and others raise in this debate. The aim of this presentation is to contribute to our understanding of how a professional ethics emerged in the late medieval period.