In the introduction, historian and philosopher of science Thomas Kuhn lays out a radically new conception of scientific discovery. Most people, raised on simplistic science textbooks, believe that scientists make straightforward, linear progress toward objective truth. But Kuhn believes that the history of science is more circular than linear in nature, and that by teaching people to look at the history of science in this way, he can help reshape the popular view of what science is and what it can accomplish. Specifically, he argues that the study of the natural world develops through a perpetual cycle of scientific revolutions, in which one set of questions and “arbitrary” perceptions are replaced by a different—though not inherently better—set of scientific beliefs.
To dig deeper into his argument, Kuhn begins by describing what he calls the process of normal science. Normal science is what takes place once one transformative insight or discovery has created a new “paradigm,” or a collection of perceptions, rules and strategies that define a certain scientific era. In normal science, scientists learn about these rules and strategies through textbooks and then work to apply them to a variety of problems.
Kuhn argues that normal science, which is what the vast majority of scientists spend their days doing, actively discourages new and original thinking. Instead, the goal of normal science is “attempt to force nature into […] the box that the paradigm supplies.” In other words, normal science entails working to prove and specify a given theory, not to alter it. Normal science is useful because it allows scientists to focus on a specific set of problems and build on one another’s work instead of constantly arguing with one another.
However, sometimes in the course of normal science, someone notices an anomaly that the theory fails to explain. As more and more people start to pick up on this anomaly, an intellectual crisis breaks out. Various researchers try to defend the existing theory in different ways, and the scientific community starts to splinter. Eventually, Kuhn suggests, one brilliant thinker has an almost instantaneous, intuitive revelation—and a new scientific theory, able to explain the anomaly, is born. Over time, this person’s theory persuades more and more scientists, and a new paradigm takes hold.
Kuhn calls the process by which one paradigm replaces another a “scientific revolution.” To illustrate this process, Kuhn provides several examples of such revolutions. For instance, Nicolaus Copernicus’s 1543 realization that Earth rotates around the sun uprooted centuries of belief in a geocentric universe. Another example is Antoine Lavoisier, whose work in chemistry suggested that combustion was not an intrinsic property of certain chemicals but rather the result of different compounds reacting with one another.
Through his examples and analysis, Kuhn draws several conclusions about the nature of scientific revolutions. First, he asserts that normal science—though it discourages new discovery—is ultimately what makes scientific revolutions possible. In order to notice an anomaly, scientists need to know what specific things to expect, and that is exactly what normal science teaches them to do.
Second, Kuhn observes that each new paradigm tries to destroy and replace the old one rather than build on it. This is why Kuhn views scientific progress as circular rather than linear. For example, ancient Greek philosopher Aristotle believed that objects had innate natures that caused them to move in certain ways. René Descartes questioned Aristotle’s theory, believing that all motion was the result of various substances bumping into one another. Most people then dismissed Aristotle’s conception—until Isaac Newton theorized gravity as an innate property, putting his followers more in line with Aristotle than with Descartes. Rather than moving in a straight line, therefore, science had moved in something like a circle.
Third, Kuhn suggests that no one scientific theory or paradigm is inherently more accurate or better than another. Rather, because each theory is the product of the “arbitrary” perceptions and questions that define its time, a paradigm shift is fundamentally a change in the way scientists see and experience the world. That is why one worldview or paradigm is almost impossible to square with another (what Kuhn terms “incommensurable”). Moreover, Kuhn emphasizes that scientists are human, and that new paradigms emerge not because they have more inherent worth, but because they are more persuasive.
To conclude, Kuhn argues that because science is circular, subjective, and based on perception, it will never reach one single, objective, truth. In fact, Kuhn suggests that no such objective truth exists.
In a brief postscript to his original text, written seven years later, Kuhn responds to critics and clarifies some of his earlier points. Specifically, he emphasizes that while his theory does have some broad applications, science is a unique field because there is more professional training and less room for disagreement or creativity than in other disciplines. Finally, Kuhn calls for more study of various kinds of intellectual communities, as these are the groups that produce most collective knowledge.