When people misrepresent their beliefs in response to social pressures, public discourse gets impoverished. Because public discourse is a basic determinant of individual perceptions and understandings, a by-product is the distortion of private knowledge. This article highlights a tendency for beliefs treated as unthinkable to disappear from society's repertoire of ideas, that is, to become unthought. Two mechanisms are identified as the vehicles of this destructive evolutionary process. The first is intragenerational: As individuals we base many of our judgments on social proof. The second is intergenerational: We tend not to think about matters our forebears have treated as settled. The argument distinguishes between hard beliefs, which are based on direct observation, inference, and analysis, and soft beliefs, which are based exclusively on social proof. © 1993, SAGE Periodicals Press. All rights reserved.