 Howard Margolis (1993) distinguishes two kinds of revolutions, depending on which kinds of problems they solve. Those revolutions that bridge gaps, he contends, differ from those that surmount or penetrate or somehow evade barriers. His focus is on barriers, a neglected topic even though it fits Kuhn’s account of cognition well. Margolis develops Kuhnian themes in terms of deeply ingrained “habits of mind.” While such habits are necessary for efficient scientific work within any specialty discipline, they constitute barriers to alternative conceptions. More broadly, deeply ingrained cultural habits of mind can close off opportunities that, according to the perspective of later generations, were staring them in the face. Margolis is struck by the apparent fact that all the materials for Copernicus’ new model of the solar system had been available, in scattered form, for centuries. No new gap-crossing developments were needed. He concludes that, rather than a gap to be bridged, the problem was a cognitive barrier that needed to be removed, a barrier that blocked expert mathematical astronomers from bringing together, as mutually relevant, what turned out to be the crucial premises, and then linking them in the tight way that Copernicus did. If Margolis’ account of the Copernican Revolution is correct, it provides an example of revolution as holistic reorganization of available materials, hence the non-piecemeal, noncumulative nature of revolutions. The developments that lead to a barrier’s removal can be minor and, as in the case of Copernicus, even quite peripheral to the primary subject matter that they ultimately help to transform. Here one thinks of a model popular with mystery writers, where an everyday observation leads to a sudden change in perspective.