PDF Summary:The Structure Of Scientific Revolutions, by Thomas Kuhn

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Science has often supported beliefs that are totally incompatible with what we believe today, and yet we haven’t changed our beliefs simply because we got “better” at science. This seems to go directly against the idea that science is simply gathering facts and building on what’s already known.

This is all leading to a shift in how older kinds of science are viewed. Instead of steps along the way to our current ideas, they might have just been products of their time, eventually pushed aside by new ideas. If that’s the case then, rather than precursors to our current scientific beliefs, they simply aren’t relevant to modern science at all.

Therefore, instead of asking how those old views led to today’s science, some historians now ask how they fit into the beliefs of their times. That is to say, they look for internal consistency in the fields rather than some overarching consistency throughout history.

When Nature Doesn’t Fit Scientific Biases

Problems often arise because accepted scientific theories are biased in some way, which leads to inaccuracies. Scientists will inevitably bring their own biases and experiences to whatever they study.

This is especially...

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Example: The Study of Light, With and Without Paradigms

For an example of this pattern, let’s look at the history of optics, the study of light. We see a series of scientific revolutions and changing paradigms going back to the 17th century: Modern studies of light are based on the belief that light is made of photons, which have characteristics of both particles and waves. However, before that model was developed in the early 20th century, most physics books taught that light was a wave. If we go back even further, scientists believed light was made of particles and looked for evidence that it puts pressure on solid objects.

However, before the 17th century, there were many competing schools of thought and no one universally-accepted paradigm. There were so many different schools of thought about light because, at the time, there were no common beliefs or understandings that they could take for granted. Due to the biases and past experiences each scientist brought, they came to wildly different—but equally valid—conclusions. Each school of thought supported itself by pointing to certain observations about the world that it could explain better than the others....

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Third, as long as there is a universally-accepted paradigm—or close enough to one—normal science doesn’t need established rules. The scientists all agree on the problems and solutions that their field has, so they’ll naturally work in the same or similar ways, as was demonstrated in the analogy to Wittgenstein's question. However, the other side of this coin is that normal science does need concrete rules when a paradigm is unstable; in other words, just before and during a scientific revolution.

Finally, normal science—even within the same field—isn’t completely uniform. There could be any number of specialties and subspecialties that, to an outsider, seem to fall under the umbrella of one particular field.

Sometimes a scientific revolution can actually be very small, affecting only a particular specialty within a field. Therefore, there can’t be scientific rules that affect the entire field, when paradigms can change for subsets of that field. The next section will go into detail about subsets within fields and how paradigms can affect them differently.

Anomalies and Discoveries

Normal science doesn’t look for anomalies or unexplained phenomena, but it often...

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A good example of this is Ptolemaic astronomy, which was the paradigm before Copernicus’s time. While it predicted most planetary movements correctly, small details such as when equinoxes would occur never quite lined up with reality.

Scientists kept making small adjustments over the course of centuries, until the system became so patchwork and so convoluted that it couldn’t possibly be right. This is a clear sign of a scientific system in crisis. Finally, Copernicus said that the Ptolemaic system had become a monster, and he rejected it entirely to replace it with his own model.

From Crisis to Theory

In the Copernican revolution and almost all others, new theories come about when normal science repeatedly fails to explain anomalies, often over the course of many years. However, while the failure takes a long time to reach the point that it can’t be ignored, the revolution often occurs relatively soon after that.

In many cases, there are only 10-20 years between the crisis point and the establishment of a new paradigm, replacing the old one. While that sounds like a long time, consider that in the previous example it took hundreds of years for scientists to...

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Scientific crises aren’t resolved by study and interpretation—that would be normal science, which by definition can’t fix a crisis. They’re resolved by a sudden understanding or change in perception resembling the gestalt shift. Scientists have sometimes described that shift like a flash of inspiration, or scales falling away from their eyes, rather than a deliberate understanding of the material.

What Happens After a Scientific Shift?

Many readers will say that what changes with a paradigm is only how scientists interpret their observations, not what they see. In other words, the observations themselves are unchangeably set by nature, and all that changes is the observers’ understanding. However, like normal science, interpretation needs a paradigm in order to exist. Furthermore, interpretation of a paradigm can’t fix that paradigm. It can only explain and expand upon it. It’s impossible for a paradigm to disprove itself; all it can do is find anomalies that eventually lead to crisis.

For example, consider a stone swinging on the end of a string. Aristotle, whose paradigm said that objects were naturally drawn to their places of rest, saw a stone attempting...

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Negative Verification

The philosopher Karl Popper suggested an alternative, which could be called negative verification, or falsification. Under negative verification, tests would be conducted with the express purpose of failing, thereby disproving a theory.

(Shortform note: To give a somewhat outlandish example, if you showed that you could drop a stone and have it fall up instead of down, you would have disproved the theory of gravity. So far, every test conducted has failed to disprove that theory.)

It’s worth noting that negative verification serves much the same purpose as anomalies: Showing fatal flaws in current ideas, thereby paving the way for new ones. However, anomalies and falsifications are not the same thing. In fact, true negative verification might not even exist.

Remember that very, very few paradigms can answer every question they’re presented with. That is the reason why normal science exists. Therefore, if a single failed test were enough to reject a paradigm, they would all be rejected almost instantly.

There would have to be some point at which the failure is great enough that the paradigm can be comfortably rejected. In other...

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In some ways the question is purely semantic: Does something progress because it’s a science, or is it science because it progresses? There is no firm answer to this, and it’s not even clear how important a question it really is.

Instead, it seems likely that the question stems from some insecurity about one’s own field, and why it doesn’t seem to progress in the same way that the established sciences do. For example, economists don’t seem to spend much time debating whether economics is a science. That’s probably not because they all agree on what science is, but because they agree on what economics is, and are confident in it.

Revolutionary Progress

In an apparent contradiction to the previous section, revolutionary science is also seen as progress, even though it doesn’t use accepted methods to solve accepted problems. In large part, this may be because revolutions must end in victory for one side or the other, and it’s unlikely that the winning side will see its victory as anything but progress.

That school’s paradigm is then written into textbooks and other educational materials, and it becomes what the next generation of scientists believes as well....

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To show how Kuhn is both right and wrong about normal science, consider the Higgs particle. The Higgs particle is a theoretical particle that would fill a gap in the current physics paradigm. Normal science is trying to find this particle.

In looking for the Higgs particle, scientists aren’t trying to discover anything new; they’re trying to find evidence to support the current paradigm. This is what Kuhn got right. However, it’s possible—even expected—that discovering the Higgs particle will open up entirely new branches of physics. This is what Kuhn’s ideas about normal science gloss over. (Shortform note: The Higgs particle—now called the Higgs Boson—was first observed in 2012, the same year this essay was published.)

Supplemental Reading

There are a number of supplemental readings you could do to better understand Structure and the author’s life:

• The online Stanford Encyclopedia of Philosophy has some great intro-level reading about Kuhn’s life and work.
• The Essential Tension is a collection of papers that Kuhn published at around the same time as Structure. It contains commentary and expansions on a lot of the same topics.
• _Black-Body...