While a large social-choice-theoretic literature discusses the aggregation of individual judgments into collective ones, there is much less formal work on the transformation of judgments in group communication. I develop a model of judgment transformation and prove a baseline impossibility theorem: Any judgment transformation function satisfying some initially plausible conditions is the identity function, under which no opinion change occurs. I identify escape routes from this impossibility and argue that the kind of group communication envisaged by deliberative democrats must be ‘holistic’: It must focus on webs of connected propositions, not on one proposition at a time, which echoes the Duhem-Quine ‘holism thesis’on scienti…c theory testing. My approach provides a map of the logical space in which di¤erent possible group communication processes are located.