Readers of this monumental study of the past four centuries--the era of modern science--have a great treat in store. Cohen, the distinguished Harvard historian of science, is a lucid expositor who states his views boldly and provides rich documentation. His explicit intention is to examine the historical meaning of the term ""revolution"" as it was used by innovators and their contemporaries at specific times and places, and how its use reflects and affects social and political currents. To some extent he regards Thomas Kuhn's Structure of Scientific Revolutions as a watershed, promoting lively debate that has affected science-historians as much as scientists. (Contemporary geologists are among the first to admit that the plate tectonics revolution fulfills Kuhn's notion of a basic shift in the paradigm.) Criteria for revolution are, however, strictly historical: 1) acknowledgment of the signal importance of the contribution by peers; 2) historical evidence that the scientific innovation was adopted; 3) affirmation by past and present historians, philosophers, and historians of science; 4) agreement among today's workers in the scientific field that the work was revolutionary. By these criteria Newton, Descartes, Harvey, Lavoisier, Darwin, Bohr, Einstein, and Freud are revoluionary figures; Copernicus, Kepler, Vesalius, and Paracelsus are not. Cohen is particularly emphatic with regard to Copernicus. Not only does he regard Copernicus as an almost slavish follower of Ptolemy, departing only in his declaration that the earth moves; but Cohen also takes to task myriad scholars for erroneously attributing to Kant the statement that he was effecting a ""Copernican revolution"" in philosophy. Cohen says that Kant was a modernist in his use of the word revolution, giving it the post-1789 meaning of a radical transformation (as in government), as opposed to earlier cyclical or ebb-and-flow notions of revolution (inherent in the Latin root and the use of the word in mathematics or astronomy). Much of the value of Cohen's text lies in the meticulous case histories of his celebrated cast of characters--embracing their writings, their style, the views of friends and enemies, and the larger cultural context. In addition to the natural sciences and medicine, Cohen also examines the status of the encyclopedists, the French positivists, Goethe, Marx, Engels, and some contemporaries of Freud. A final chapter discusses the joining of today's notion of revolutions as radical, with the idea of conversion--as when a scientist staunchly opposed to an innovation suddenly ""sees the light."" (Cohen offers more than one area where scholarship in needed, e.g., explorations of the psychology of revolutionary thinkers, analysis of how concepts are transformed.) A wonderful excursion into past and present revolutions that enriches the literature and should in itself generate some fine mind-turnings.