Hailed by the poststructuralist left, wielded by feminists and fundamentalists alike, and hated by most practitioners of the field it purports to explain, Thomas Kuhn’s The Structure of Scientific Revolutions is perhaps the single most important work on the nature of rationality since Descartes’ Meditations .

To support this assertion, a little background. In the years immediately following World War II, most philosophers of science were logical positivists who believed that science involved two stages: first empirical research, then logical analysis of the results. Experimental science provided the raw data, while philosophers analyzed that data and clarified the theories used to explain it. Any statement that could not be verified by science””The universe has a first cause” or “God is infinitely wise,” for example”these philosophers considered meaningless. Only by rigorous conceptual analysis and the rooting out of all unverifiable statements could scientists achieve certainty.

In the late 1940s, Kuhn, then a doctoral student in physics at Harvard, was asked to teach a course introducing nonscientists to the practices of science. As he later wrote, “To my complete surprise, that exposure to out“of“date scientific theory and practice radically undermined some of my basic conceptions about the nature of science and the reasons for its special success.” As a result, Kuhn turned his attention to the history of scientific revolutions”those times when one widely held scientific theory is challenged on a fundamental level by another and eventually replaced. The best“known revolutions are associated with Copernicus, Newton, and Einstein in physics, but equally fundamental revolutions occurred with Lavoisier in chemistry, Maxwell in electromagnetism, and Planck in atomic theory, among others. Kuhn’s studies revealed that at the time these revolutionary theories were proposed, there was no rational way to determine which theory was correct.

Although Copernicus simplified certain astro nomical calculations by suggesting that the sun”not the earth”was the center of the universe, he did not explain the observed movements of the stars any better than the geocentric theory he opposed. Most astronomers were willing to treat his theory as a mathematical shortcut, a way of looking at reality from a different perspective, but they did not actually subscribe to it as fact. Yet some astronomers, Kuhn found, were also Renaissance Neoplatonists and Neopythagoreans, believing that mathematics was the basis of all truth and beauty. For them, the simpler calculations that Copernicus’ theory permitted were enough to make a strong case for its truth. Despite what the textbooks say, it was not the logical consequences of astronomical observations that persuaded the first Copernicans; they believed the new theory because it fit with their peculiar religious theories about astrology and numerology.

Kuhn’s own experience as a physicist bore this out. Scientists, like other people, have to make decisions about what projects to pursue and how to allocate their time. Younger scientists have to decide which research projects will get them tenure or grants or control of a laboratory. Older scientists have their reputations to defend. In addition, scientific articles are taken seriously only if published in certain journals, so research is constrained by what the editors of those journals will accept. What the scientific community takes up depends on all sorts of personality conflicts, nonrational prejudices, logistical prob lems, and what insurance companies call “acts of God.” Kuhn realized that because science textbooks were more useful if they just taught the conclusions and methods of science without all the false starts and theories discarded along the way, the story science told about itself ignored the ambiguity of its actual practice.

Kuhn thought that these sociological factors explained why scientists would not divide up into the warring schools that marked other disciplines. Most of “normal” science consisted of problem“solving rather than research into fundamentals, because the pedagogy and institutions of science discouraged researchers from questioning the principles of the current scientific paradigm. Moreover, Kuhn argued (and this forever endeared him to postmodernists), a scientist’s indoctrination into the reigning paradigm was usually so complete that it affected his observations and experiments. Scientists don’t just conduct experiments to collect raw facts, but to prove or disprove some hypothesis. The hypothesis determines which variables to isolate and which to ignore, and they limit the acceptable explanations of the results.

Science is successful because scientists deliberately restrict their vision and their imagination in order to see some particular thing better. Kuhn showed that in doing so science also bound itself to a set of assumptions that it did not even recognize as such. Enough surprising or anomalous results can make the assumptions of a long“held theory visible again, but only if some free“thinking scientist pulls back from his hypothesis long enough to look at the big picture. The conservative culture of science discourages such free thinking as unscientific; Kuhn’s achievement was to show that many of the great scientists of paradigms past were “unscientific” by this standard.

The Enlightenment view of reason defined rational arguments as those so clear, and consequently so certain, that no sincere person could reject them without being suspected of perversity. Kuhn showed that, within a paradigm, this view holds most of the time. But brilliant scientists who engage in different research programs based on different assumptions”i.e., who, in Kuhn’s terminology, inhabit different paradigms”can have fundamental disagreements without ceasing to be brilliant scientists. We know this holds in other disciplines”literary criticism, for example. It is just that we believed that scientific reasoning, reasoning at its best, would not be subject to the same uncertainties. Kuhn, by pulling back the curtain on real scientific practice, showed scientific reasoning to be just a species of dialectic, perhaps more disciplined than others, but not in principle different or indubitable.

The exciting result is that scientific reason can now be seen to be of a piece with the other great forms of dialectical reason in history: the Greek dialectic of Plato and Aristotle and the Scholastic disputationes of Aquinas. All of these marry a high level of logical rigor and an all“out pursuit of the truth with a method that appeals to an entire community to judge which arguments are the most sound. If reason is communal in this way, then Descartes’ Meditations and Enlightenment epistemology in general are flawed. Sitting alone in his room, someone might be unable to tell his dreams from reality, but when engaged in a multifaceted discussion with very different sorts of people, one must concede that there is more to the reality he experiences than his imagination can contain. And if reason is communal, then it involves not just information and logic but rhetoric and poetry, charity and sensitivity, self“confidence and mutual respect. Most of all, it involves creating and sustaining a culture of inquiry, a high“level conversation that can include many voices without losing its direction. Kuhn called it a paradigm. Hans Urs von Balthasar compared it to a symphony.

Around here we call it A Journal of Religion and Public Life.

Daniel P. Moloney is Associate Editor of First Things .