Science  /  Book Review

What Was the “Paradigm Shift”?

When Thomas Kuhn coined the term, he wasn’t referring simply to “out of the box” thinking.

The Structure of Scientific Revolutions, in other words, is one of those books that everybody knows but doesn’t read, or reads once and shelves. On rereading my copy, neglected since a first-year graduate seminar in the history of science over 25 years ago, I was struck by Kuhn’s insistence on the power of historical research to puncture idealized claims of scientific progress. Paradigms and normal science? Sure. But the truly radical idea here is that outsiders—in this case, historians—can offer better insight into the inner workings of a profession than the practitioners themselves.

What, exactly, is a paradigm shift? In Structure, Kuhn defines a scientific paradigm through its relation to what he calls “normal science.” A mature scientific community, one that is relatively secure in its methods, intellectual assumptions, and choice of problems, is operating in a period of “normal science.” Collectively, those rules and standards for scientific research constitute “shared paradigms.” These shared paradigms lay a path for scientific communities to work efficiently, allowing individual scientists to focus on the “mop-up work” of collecting data and solving puzzles suggested by the operating paradigm. 

Over time, however, this routine puzzle-solving work will generate “anomalies” that violate the expectations established by the paradigm. At first, scientists who encounter such anomalies tend to assume that they have made some sort of experimental error. Those who insist on the correctness of their divergent findings may be considered cranks. An abundance of anomalies, once acknowledged by the community, throws that community into crisis. When the crisis cannot be resolved by tweaking the existing paradigm, a competing interpretation that casts the data in entirely new ways may gain ascendance. This switching of the paradigms constitutes a scientific revolution. Scientists might reject phlogiston in favor of oxygen, for example, or embrace heliocentrism after centuries of Aristotelianism. 

Kuhn famously described these competing paradigms as “incommensurable.” A belief in phlogiston is simply incompatible with the existence of oxygen. You cannot simultaneously believe that the sun orbits the Earth and that the Earth orbits the sun. Drawing on mid-twentieth-century psychology and theories of mind, Kuhn compared these shifts in attitudes to gestalt shifts—the kind of change in perspective that turns an image of a rabbit into a duck. In perhaps the book’s most controversial chapter, Kuhn argued that changing paradigms changes nature itself, or at least the way scientists perceive it. Change a paradigm, change the world.