In "Paradigms and Barriers" Howard Margolis offers an innovative interpretation of Thomas S. Kuhn's landmark idea of "paradigm shifts, " applying insights from cognitive psychology to the history and philosophy of science. Building upon the arguments in his acclaimed "Patterns, Thinking, and Cognition," Margolis suggests that the breaking down of particular habits of mind--of critical "barriers"--is key to understanding the processes through which one model or concept is supplanted by another. Margolis focuses on those revolutionary paradigm shifts-- such as the switch from a Ptolemaic to a Copernican worldview--where challenges to entrenched habits of mind are marked by incomprehension or indifference to a new paradigm. Margolis argues that the critical problem for a revolutionary shift in thinking lies in the robustness of the habits of mind that reject the new ideas, relative to the habits of mind that accept the new ideas. Margolis applies his theory to famous cases in the history of science, offering detailed explanations for the transition from Ptolemaic to cosmological astronomy, the emergence of probability, the overthrow of phlogiston, and the emergence of the central role of experiment in the seventeenth century. He in turn uses these historical examples to address larger issues, especially the nature of belief formation and contemporary debates about the nature of science and the evolution of scientific ideas. Howard Margolis is a professor in the Harris Graduate School of Public Policy Studies and in the College at the University of Chicago. He is the author of "Selfishness, Altruism, and Rationality" and "Patterns, Thinking, and Cognition," both published by theUniversity of Chicago Press.