Disagreeing despite the Data: The Destruction of the Factual Commons examines the pressing problem of factual disagreement between social groups, suggesting that the belief segregation underway in the United States may be irreversible. David Apgar draws on the work of twentieth-century philosophers of science and language—especially Popper, Wittgenstein, and Davidson—to identify three requirements for factual agreement to be possible at all: a pervasive habit of checking assumptions, densely connected communities, and projects that straddle those communities. The growing refusal to test assumptions and individual isolation can be remedied by critical thinking and community building. Factual agreement between groups is impossible without shared projects or other meaningful interaction, however, and a large part of American society has insulated itself from the rest. Without shared projects, communities lose the ability to tell whether they agree or not regardless of the words they use. Disagreeing despite the Data looks at the destructive effects of belief segregation with similar roots in several dissimilar developing countries on a path wide enough for richer ones, like the United States, to follow.