The Structure of Scientific Revolutions

To some extent, Kuhn argues, “when paradigms change, the world changes with them.” Though scientists are not literally transported to a new planet, they begin to understand the world in radically new ways. Again using the example of the Rorschach test, Kuhn suggests that each new scientific student’s perception of the world “is determined jointly by the environment and the particular normal-scientific tradition that the student has been trained to pursue.” When a paradigm shift occurs, therefore, part of the student’s world changes.

Using the field of gestalt psychology, Kuhn points out that once people have seen the world in a new light, it is almost impossible to revert to their old perceptions of it. However, bridging the gap between historical study and psychology presents an interesting problem for Kuhn’s own work; he feels his work on this junction is not yet complete.

In particular, while subjects of psychological experiments are able to acknowledge that the shift in reality is really a shift in their perceptions, scientists tend not to do so. Or, as Kuhn puts it, “looking at the moon, the convert to Copernicanism does not say ‘I used to see a planet, but now I see a satellite.’ […] Instead, a convert to the new astronomy would say ‘I once took the moon to be […] a planet, but I was mistaken.’”

Kuhn also discusses the example of Uranus, a celestial body that was the subject of much debate among astronomers. Throughout the 1700s, many different people observed Uranus as a star, because they did not note its motion. When one scientist finally saw it move, he believed it to be a comet. After several months of unsuccessful efforts to assimilate Uranus into existing comet theory, scientists accepted that it was a planet. And suddenly, right after accepting Uranus as a minor planet, astronomers began to see minor planets and asteroids everywhere. Kuhn argues here that in changing their beliefs about Uranus, scientists looked up at the sky and noticed different things. 

The most dramatic example of this shift in perception occurred with Galileo. Aristotle explained the pendulum by saying that heavy objects naturally fall, and the object at the bottom of the pendulum is merely falling slowly (because its chain holds it back). Galileo, however, began to take in the pendulum as a “swinging body” that repeats the same motion over and over again. This new way of seeing the pendulum allowed Galileo to come up with many critical scientific ideas about weight, incline, and velocity.

Kuhn notes that Galileo’s “shift of vision” did occur in part because of “his individual genius.” But at the same time, Galileo was drawing on the work of earlier scientists, who had come up with something called “impetus theory.” His knowledge of impetus theory allowed Galileo to see the pendulum as something separate and specific, not just a “swinging stone.” It was a knowledge of impetus theory, then, that allowed Galileo to see the world differently and therefore to conceptualize the pendulum.

But if Galileo had gleaned insight from impetus theory, which was compatible with an old paradigm, Kuhn believes that his new way of seeing was ultimately the result of a “lightning flash” change in perception. In other words, if normal science leads to a crisis, it can never itself lead to a new paradigm. Instead, new paradigms are only reached by these almost intuitive reassessments of the natural world. 

Kuhn then pauses to consider why this focus on a given scientist’s “immediate experience” is so necessary. For centuries, Western epistemology has argued that sensory experience is “fixed and neutral.” Kuhn acknowledges that he does not believe sensory experience is really so simple—but that he does not have an alternative explanation. So, while Kuhn is confident that scientists’ draw on their paradigm’s assumptions for even seemingly factual observations, he lacks a theory of perception to describe why this is the case. Kuhn thus hopes for a paradigm shift in both psychology and philosophy that would help him explain this non-fixed, non-neutral form of perception.

Stepping back, Kuhn notes that “neither scientists nor laymen learn to see the world piecemeal or item by item.” For example, a child learning to call her mother “mama” is also learning about gender and family structures in general. Paradigms do not determine single facts or experiences, and so paradigm shifts affect many ideas and observations at once. 

Fascinatingly, then, science in a new paradigm involves many of same techniques, tools and terms as science in the old paradigm did. However, even when the same techniques and technologies are applied in a new paradigm, they are understood to reveal dramatically different information.

To exemplify this, Kuhn discusses the scientific revolution caused by John Dalton. For much of the 18th century, chemists worked under the paradigm of affinity theory, which dictated that certain substances dissolved in others because of an innate attraction. At the end of the century, some chemists began to realize that a select few chemical mixtures had fixed proportions of their various ingredients. But no coherent theory was created, and the discipline started to fracture (and appear more like pre-paradigm science).

Dalton was ultimately the one who overturned affinity theory with his famous atomic theory. However, Dalton identified as a meteorologist—in his initial tests, he was not using affinity theory at all. Because he came from a different discipline, Dalton was able to craft the law of fixed proportion (all atoms will bond to one another in simple, whole-number ratios). At the same time, Dalton’s theory allowed him to claim that any time ingredients in a compound were not in this kind of ratio, the compound was not chemical.

Though some chemists were deeply opposed to Dalton’s view, this new paradigm quickly proved more useful and efficient. Since even skilled scientists had been conducting their experiments with different goals and ideas (under a different paradigm), Dalton had very little evidence for his theory initially. Instead, that evidence came after—future scientists “beat nature into line,” and when they were done, “the percentage composition of well-known compounds was different. The data themselves had changed.”